Browser Game Slowdown

I recently built my first computer, and so far it's worked out well. However, I've experienced some massive slowdown in browser-based games (especially those with Java applets involved), but I receive great gameplay on other games (I've tried Guild Wars, for example, with no issues). I can't seem to locate the problem, so I was hoping you guys could help me out.

Thanks!

I've tried that with no success.

I'm trying to play this game called RuneScape (among others) and when the login screen pops up when everything's fully loaded, the graphics seem fine for a few seconds. Shortly after, though, the refresh rate seems to slow down to 1 frame every 5 or so seconds and the game becomes unplayable. Does anyone have any ideas as to what the cause of my problem is?