Mac goes to sleep every 4 seconds while steaming video

The Problem:
Every time i want to stream a video from the internet, i.e. surfthechannel and youtube, my computer will suddenly go into SLEEP MODE, and then continues to do so after i wake it up, every 4-10 seconds. Sometimes it continues this after i have closed the browser completely. This has been going on for about 3 weeks.

Here's what i've done so far:
>Of course the very first thing i do is check system prefs and look at all my
setting.
> I restarted my computer, sometimes it'll get to my desktop, and then go
to sleep immediately.
> Changed Browser, i used SeaMonkey, Safari, and Firefox, it occurs
with all of them.
> Then i backed up all my programs and files, and reinstalled Leopard. That
didn't help.
> The i spent 3 hours on the phone with Apple and all they had me do was
restart my computer using the refresh keys and try safe mode.
>THEN! i brought it to an apple store, where they kept it overnight!!!! THEY DIDNT FIND ANYTHING WRONG WITH IT!

IM GOING INSANE! INSANE INSANE INSANE INSANE!!!!

This only happens at your home? In other words, when you're using your connection? Are you using Airport wireless or ethernet?

If on wireless, try attaching an ethernet cable and see if it does the same thing. Also try using the machine from another location to see if it goes to sleep. Let us know.

If you go insane in the meantime, have your keeper post the results.

Regards.

I'm using a comcast modem connected to a Linksys router. There arent any other two companies id rather see suffer from our current "economic crisis".

While overnight at the Apple store, they did tell me they've "tried everything" to recreate my problem but found nothing. I dont understand how my internet connection would effect my computer going to sleep. But ill try what you suggested, thank you!

Also i should mention that often when restarting my computer, it somehow loses connection to the internal speakers, it wont even acknowledge they exist and when pressing the volume keys, a circle with a line diagonally through it pops up under the picture of the speaker.

Connecting straight to the ethernet didnt make a difference.