On my Tosibha Qosmio X775-Q7170 in my upstairs bedroom my wifi signal is 100%. I had bought a repeater because the signal was very low, 30-40% and I was getting some serious lag, and my connection was getting lost.
Although between 9pm and 1am my internet is absolute garbage, just on the wireless. Hardwired connections are still at their normal speeds with 0 packet loss.
The funny thing is, I can skype on that laptop when I get this strange lag, but it's not MY webcam that gets all laggy and pix elated, it's my girlfriends. This leads me to beleive that my downstream is being pooped on somewhere down the line and I can't figure out how to fix it. My roomate is the one with all the knowledge of the router, I don't know anything about routers, but I know it's a linksys ,and it's fairly new.
Is comcast just pure garbage? Sadly there's nothing else in my area -_-
All the drivers on the laptop are up to date, and the firmware on the router is it's newest. This has been happening for several months now, even after the new linksys router.
Signal strength is not the same as signal quality. You could very well be getting interference and still have plenty of strength. What your testing proves is it is not your internet connection since it works wired it is one of the wireless connections.
With a repeater in the middle you now 2 wireless connections to trouble shoot. Since it is a certain times of day it is most likely another person running wireless. If it is a neighbor your only choice is to try to change the channels to avoid it.
It could also be as simple as other users in your house, wireless degrades quickly with lots of other people...and those nearest the main router will always win over those further away.
During your 9pm to 1am issue, who else is using your internet? For example, if like you stated, "My roommate is the one with all the knowledge of the router", then is your roommate also using the internet at this time. For example, that person could setup QoS for his device and therefor takes priority on the bandwidth and leaving you the crumbs. conspiracy theory begins...
I think john-b691 is right on with the cause being WiFi interferance and trying diff channels on the router setup might be the fix. If you can't run a network cable to your location, maybe invest into a Powerline adapter kit to add a ethernet port to your room.
Example: TRENDnet TPL-306E2K for about $66. So if your wired solution is not affected, then maybe try to get a "wired" solution.