Multiple Possibilities...
There are several things that could be causing these symptoms. Of course, heat is an obvious possibility. I would also check your System Information utility to view how much RAM is actually available. RAM loss is a common issue that causes the choppiness & the flash-heavy site lag.
Another issue that would be useful to check is the video card. Make sure that the fan is still running freely on it. Also, run a video check. Most video cards these days come with the software for these checks. If you don't have any, you can download some here.
That's where I'd start. If that doesn't fix it, just post back and I'll try to help you further.
There are several things that could be causing these symptoms. Of course, heat is an obvious possibility. I would also check your System Information utility to view how much RAM is actually available. RAM loss is a common issue that causes the choppiness & the flash-heavy site lag.
Another issue that would be useful to check is the video card. Make sure that the fan is still running freely on it. Also, run a video check. Most video cards these days come with the software for these checks. If you don't have any, you can download some here.
That's where I'd start. If that doesn't fix it, just post back and I'll try to help you further.