That the slow down happens after a while suggests that its a temperature thing.
I'll keep an eye on that and see...
The drive reving I would guess is swapping happening re the browser needing to page memory in or out. Does it happen if you're not using X but only console? Its spin and pause is to save your battery. If you're on AC there is probably a way to have it not spin-down (uses morejuice but easier on the drive motor).
I'll look into this. Thanks for the suggestion. Cheers, Tyle