That PSU was "fair" when it was new, but now it's like 9 years old. Likely it is the problem or at least part of it. I'd replace that first and go from there. Keep in mind, even though there are other possibilities, if you have a PSU issue it's hard to determine if something else is actually faulty because the power supply can mimic pretty much any other hardware failure even though that hardware is actually not bad, or in some cases can actually CAUSE that hardware to fail in the first place, so replacing the failed part then becomes just a temporary fix as will be likely to fail again if there is still a problem with the power supply.
9 years is a long time for a product that came with a 2 year warranty. The platform used in that PSU was not very exceptional, just merely ok. It was not a bad unit, but nothing about it cried out quality either except maybe a few of the actual results during testing but there were a lot of unknown parts used on it as well so longevity was something not really expected from it.
I'd replace it FIRST, and go from there. At least then you KNOW it's not problem and since you've already addressed all of the easier, more common issues, this is a likely suspect right now.
I don't know what country you reside in, and I know that sometimes it's hard to come by good units in some regions, but when possible, when it comes time to get that PSU, I'd stick to the following if you can.
Higher 80plus certification doesn't mean anything, UNLESS it's on an already known to be high quality PSU platform. For example, a Seasonic Prime platinum unit is going to be a better product than a Seasonic Prime Gold unit, because we already know the Prime platform is very good, and platinum efficiency along with it shows there are some improvements internally to account for the higher efficiency.
In a case like that, it might be worth it. It's likely the unit will create less heat, it will probably have better performance in regard to ripple, noise and voltage regulation. It might shave a few pennies, or dollars, off the electric bill over the course of a year.
Other than that, it is not going to perform any better than the same platform with Gold efficiency. On the other hand, just because a unit has Titanium 80plus ratings doesn't mean the unit is any good at all. For example, there are Raidmax units with Titanium efficiency and I wouldn't trust one of those to power a light bulb. There are a lot of units like this out there.
80plus only has relevance if the platform is already known to be a good one.
Seasonic. Just about anything made by Seasonic is good quality for the most part. There are really no bad Seasonic units and only a very few that are even somewhat mediocre. They do make a few less-good quality OEM style units, but mostly those are not going to be units you come across at most vendors, and they are still not bad. Also, the S12II and M12II 520 and 620w units are older, group regulated models. At one time they were among the best units you could buy. Now, they are outdated and not as good as almost any other Seasonic models. They are however still better than a LOT of newer designs by other manufacturers.
The Seasonic 520w and 620w S12II/M12II units CAN be used on newer Intel platforms, if you turn off C6/C7 in the bios, but I'd really recommend a newer platform whenever possible. Prices are usually pretty good on those though, so sometimes it's worth accepting the lack of DC-DC on the internal platform. Higher capacity versions of the High current gamer are not based on that platform, so they are fine. Those being the 750w and higher versions. Antec Edge units are ok too, but reviews indicate that they have noisy fan profiles.
Corsair. The CX and CXm units are ok as a budget option, but I do not recommend pairing them with gaming cards. The newer 2017 models of CX and CXm are better than the older ones, so if it specifically says 2017 model, or it has a capacity other than an even 100, like 550w, 650w, 750w, etc., then it's likely at least better than those older ones. Aside from that, any of the TX, RMx, RMi, HX, HXi, AX or AXi units are good. Those are listed from best to worst, with the best being the AX and AXi units.
Antec. The True power classic units are made by Seasonic, and are very good, but are not modular. The High current gamer 520w and 620w, or any other PSU you see on the market that is 520w or 620w, are also made by Seasonic, based on the S12II and M12II platform for modern versions, and are pretty good units but again they are an older platform that is group regulated so if you go with a Haswell or newer Intel configuration you will want to avoid those because they do not support the C6/C7 Intel low power states.
The Antec High current gamer units that are not 520w or 620w, the 750w and 850w models, are very good and are not the older design. There are however older and newer HCG models, so exact model number will likely be a factor if choosing one of these however both the older models and the newer models are good.
Super Flower. They are like Seasonic and they make power supplies for a variety of other companies, like EVGA. Super Flower units are usually pretty good. I'd stick to the Leadex, Leadex II and Golden Green models.
EVGA. They have good and bad. Bad are the W1, N1, B1, B3 (All models except the 650w model) and G1 NEX models. Good models are the B2, B3 650w, G2, G2L, G3, GQ, P2 and T2 models.
FSP. They used to be very mediocre, and are a PSU manufacturer like Seasonic and Super Flower, although not as well trusted based on historical performance. Currently the FSP Hydro G and Hydro X units are pretty good.
I would avoid Thermaltake and Cooler Master. They do have a few good units, but most of the models they sell are either poor or mediocre, and the ones they have that ARE good are usually way overpriced.
And most of the models I have linked to the reviews of at the following link are at least good, with most of them being fantastic.
http://www.tomshardware.com/forum/id-3612443/power-supply-discussion-thread.html
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