Possibly infected with a security threatening virus

I have a feeling there are some issues with my computer that some evidence suggests is caused by a virus. I first noticed issues about a month ago when my CPU usage would randomly spike and if I was playing a game at the time, the fps would drop to anywhere between 1-5 for usually just a minute or 2 but on occasion it has lasted between 5-10 minutes.

At first I wasn't sure if it was a virus or just my computer being slow. I've run countless virus scans and rebooted just as many. The reboot will solve the issue but in the time it takes to reboot, it would have ended by itself and it isn't a fix because it will happen again after a short period of time.

It isn't just when I am gaming, it could be when I am on a skype call and the quality and latency drop or if i am watching a video where the picture and sound get choppy.

I play RuneScape and a few other non hardware intensive games from Steam. Since Runescape runs off Java, if I look at my task manager, it shows that java is taking usually 60% or more of my memory and if it is a steam game, it will show that game taking more than it normally would.

I'm now suspecting it is caused by a virus though because without me sharing my password, someone was able to get onto my RuneScape account and access my bank, which has a pin code you have to click on numbers that are in random spots. I did not share that information with anyone.

Any help would be greatly appreciated. I would love to be able to feel safe rebuilding my account and not have random slowdowns while doing so.

I asked someone who helped someone else with a similar issue and i was told to post in malware thread