MacBook Pro :: System Hangs With Colossal Hard Drive Activity

Been having a problem with my MBP recently, it's the 2008 model btw. This happens like 3 or 4 times a day it usually starts with me trying to change spaces and nothing happening, and the dock won't pop up when I mouse over the bottom. After doing this for say 30 seconds or so the active app will freeze too. I'll still be able to move my mouse it becomes unresponsive. iStat menus tells me that the hard drive is reading and writing at like 5 or 6 Mbps, and the the dock is taking up 200 or Mbs of memory. all the while nothing is happening. after a minute or two it come back slowly like I'll be able to scroll very jumpily for a bit and then the dock'll pop up and I'll whizz round the spaces I keyed in etc.

This is starting to get really annoying, I've tried resetting PRAM and the SMC thing (or whatever they are, the things you do when you turn the laptop off) and repaired the permissions etc. The only thing I can think of doing is a complete reinstall, but that takes a fair amount of effort so I was wondering if you guys had any ideas what's going on. On a separate note my laptop also does this weird sleep thing when I close the lid, it goes to sleep, then comes on again and there's HD activity and then goes to sleep again and so on and so on. This is pretty irritating because the laptop's roasting hot if you leave it like this for a while and it drains the battery. Meaning I've got to shut it down completely if I want to put it in a bag to go somewhere.