Macbook 'seizing' every 5 seconds

Question

Hi Chaps,
I'm running OS X 10.8 on the following spec:
  Model Name:          MacBook Pro
  Model Identifier:          MacBookPro5,5
  Processor Name:          Intel Core 2 Duo
  Processor Speed:          2.53 GHz
  Number of Processors:          1
  Total Number of Cores:          2
  L2 Cache:          3 MB
  Memory:          8 GB
  Bus Speed:          1.07 GHz
  Boot ROM Version:          MBP55.00AC.B03
  SMC Version (system):          1.47f2
  Serial Number (system):          W89****66E
  Sudden Motion Sensor:
  State:          Enabled
Graphics Card: GeForce 9400M
The problem is that, in some circumstances, every 4-5 seconds the computer slows almost to a stop for about 1/2 a second then returns to normal speed. This is visible on any application using graphical visuals. It can be seen on the screensaver, games (such as diablo 3 and WoW, much to my son's disgust) and the visuals in photo booth have been choppy. It does not affect sound at all. It doesn't seem to affect YouTube. The effect is a bit like an egg rolling, ie fast fast fast slooow, fast fast fast slooow.
I have repaired the disc and performed SMC and PRAM resets. None of this has helped.
Any further advice?
<Edited by Host>

Answer 1

How much free hard drive space do you have? You want around 10% or more free for systems usage.
See this FAQ on the spinning beach ball ... while you're not describing that exactly, you may find some pointers to solve the hesitations.