CPU Overheats to a Blackscreen

I have a AMD FX-4300 processor with the heatsink it came with, I personally did not build this pc, it was prebuilt from cyberpowerpc. "CyberPowerPC Gamer Ultra GUA 380" (I have only had this pc for about a year and a half after buying it brand new). So about 3 or 4 weeks ago, I was just sitting on my computer watching a youtube video and it blackscreened, I didnt think much of it so I turned it back on, it started running extremely slow, so I restarted it. It was fine for about 2 days or so, then it blackscreened again, turned it back on, had to restart it because it was running super slow. It was fine for about an hour, then it blackscreened, I turned it back on, it blackscreened insantly after booting up. I left my pc off for about a day, then took it to bestbuy and asked geek squad about it, the guy I talked to installed "CPUID HWMonitor" so I could possibly identify the issue. I did almost instantly, my processor was overheating extremely bad. I kept watch on it for a week, then took it back and asked the same guy what I could do, he told me to try and reapply the thermal paste. So, I reapplied the thermal paste, it was keeping it cooler for about a week or 2 but it was still slightly overheating, not enough to blackscreen it though. Yesterday, it blackscreened again, I turned it back on and opened the monitor program and it was overheating buy 45 degrees fahrenheit. I ended up turning the pc off and opening it up, my first thought being, maybe I messed up with the thermal paste, I was partially correct, I put on a bit too much, so I simply removed the extra paste, and turned the pc back on. It was still overheating a bit, so I decided to look at the cpu fan, it was dusty, so I cleaning it, that didnt fix it. Its not only my CPU overheating, my heatsink gets extremely hot to, to hot to touch. I have been noticing that my cpu fan stays at 25% speed, it sometimes changes, but mostly stays at 25%. That might be the issue, im not sure. Someone please help me with this because I am currently on the edge of selling my pc and getting a new one because of the issues I have had with this pc.
Thanks.