Many strange and annoying issues (mainly videos and audio).

Hello.

As of today I've been having some extremely annoying issues.
About six minutes into a YouTube video, I noticed that the video started lagging / freezing. It could pause for two seconds, then lose audio, then work as normal, then pause etc. etc.
I restarted my computer like usual, but kept having the problem.
Switching from Chrome to Safari (with newest Flash) did not help.
I have the newest software update to Snow Leopard.
I later tried listening to music. It would start doing the same thing, pausing the music, and jumping in and out of headphones.
I decided to see what would happen if I plug out the headphones, but that gives me even more issues.
I become unable to change the volume. The little icon with the speaker turns grey, and when I press the "change volume" buttons nothing happens.

I've been looking at the activity monitor, trying to find strange processes, but I don't see anything out of the usual. The last app I installed was MKVmerge or something, but that was two days ago.

I have an iMac from 2010. 4GB's RAM, i3 processor.
Still running Snow Leopard (like I said previously, I think).
I've done everything I thought would help, but it didn't get me anywhere.

Please, help if you can.

 

When I had the activity monitor opened, I noticed this:



This happens when I watch videos (or during any usage of Flash, really).
What can I do to stop this? Flash seems to be the problem here.

 

Sorry, it was a picture of my activity monitor.

Anyways, I reset my computer back to a time machinge backup from last year.
After a week, the problems returned, although in smaller scale.
When I watch videos, or do anything, really, weird stuff is going on.
I'm watching a 17 minute YouTube video. Imagine that someone would restart Flash while you're watching it. The video stops for a short while, but enough to bother you. It's done it about four times during those 17 minutes.
It loses its audio, and all audio on the computer entirely.
I was watching a YouTube video earlier, and all of a sudden the computer loses its audio, and it's gone for five minutes. I performed a restart, still had the issue. Restarted it again, and it went away.
Some of the stats are confusing in the Activity Monitor. Flash is up to about 70% while watching 720p, and about 20-40% when watching 360p.

Google Chrome is currently using 128MB.
There are three Google Chrome renderer, one at 250MB, one at 100MB and one at 50MB.
Kernel Task is at 200MB memory usage.
Flash is at 275MB.

 

Nope.
I got very reasonable values when using a different user. Google Chrome was at around 8% and Flash was at 25% (720p video).
I get good values in Safari, but since I am still using Snow Leopard I don't feel comfortable using an outdated version of the browser.
I'd rather not have to update my OS.