mouse and keyboard freezing

Hi!

Recently a problem with my macbook pro started
(2.33 Ghz, 3 GB 667 MHz DDR2 SDRAM)

Every now and then the mouse and keyboard is freezing for between 15-60 seconds before it kicks up again. It often starts with the fan is increasing. If Im on the internet for example and there are some moving content on the website that still moves so the whole computer doesnt seem to freeze. If Im listening to music through for example spotify the music works a maybe 10 seconds more (probably whats buffered on spotify)

summon it up:

symptoms:
- happens often in the same time as the fan is increasing
- mouse and keyboard freezing
- moving internet content still moves
- spotify works 10 seconds more.

what I have tried:
- opened up the macbook and cleaned it from dust
- lifted up the mac (if it should be a heating problem) so air can pass underneath
- used onyx to fix the mac

Other information:
- the temperature on the CPU is around 60 but sometimes up to maximum 90 degrees
- I installed last summer a memory so increased the ram from 2 to 3 GB. but it worked without any problems until recently

any guess what it can be?
or suggestions to solve it?

/ Perik