Sometimes laggy after video

[oborysm]

My laptop is pretty snappy with Mint 17 Cinnamon, but sometimes it unexpectedly goes into a mode where it is very laggy, the kind of laggy where you click a menu item and wait five seconds for it to open. This seems to be triggered by playing videos in the browser, but not always although I haven't figured out which ones. Sometimes a video is playing nicely, and then it will suddenly the frame rate will start slowing down until it approaches zero, while the audio will continue just fine. It even happened once without video, while displaying a whole page of Google image search results.

In this mode the whole system is slow, not just the browser. Restarting the browser does not fix it. The only thing that fixes it is a cold reboot, not a warm restart.

This started in the previous installation of Mint 16, so I did a clean wiped-disk install of Mint 17, but it continues. The laptop is not too shabby, a Dell e6400 Core 2 Duo with 4GB RAM and the latest nVidia drivers. I installed and ran MATE but that didn't help either.

One possibly unrelated issue: Shutting down from the menu doesn't lead to a power-off, it just closes everything and seems to suspend, but the power stays on.

What should I be looking for? I am thinking some buffer is filling up and not being cleared properly. When it becomes laggy RAM and swap appear to be working according to top, and I don't see any unfamiliar processes.

[oborysm]

I meant "RAM and swap appear to be working according to top..."