Cache problems in IE8 and FF3.5.4

Hello,

simple things first: Excuse my bad english, i´m from germany.

My problem:

Two weeks ago, i updated my PC. Everything is fine, except when playing videos,

the system cache is rising fast until my browsers run very slow and the whole system stops/crashes.

IE8 is running a bit more stable than Firefox 3.5.4.

First thougt: Virus. I ran two scanners (Kaspersky and Antivir), then Ad-Aware. Nothing.

Next steps: Hijackthis and ccleaner. Everything is fine. Problem still occurs.

I´ve been around a couple of forums in the last weeks, nothing worked.

I think it´s got something to do with flash, but can´t figure it out.

Any Ideas?

n4gel