Mac hangs for 20-30 seconds; over and over

MacBook Pro (2008, I think, 4 GB RAM), OS X Snow Leopard. Starting a few weeks ago, it began to stall for 20-30 seconds, again and again, every 3-4 mouse clicks. Does it in all applications. A few actions will go normally, then suddenly, I'll click and the Mac will do nothing for 20-30 seconds. Usually lasts very close to 20 seconds (which makes me think it is waiting for something which times out). Often there is a spinning beach ball. Cursor still moves and I can grab and move a window; sometimes I can click on another window to bring a different app to the front; but I can't actually do anything. Then after the 20-30 seconds, it resumes as if nothing is wrong, until it happens again, a few actiona later.

What I have tried:

- Getting rid of all extraneous apps, pref panels, e.g. LogMeIn, Hamachi, Logitech keyboard software, Kensington Mouseworks, Growl, etc.
- Disk Utility's FirstAid found no disk issues, several times
- unplugged everything and ran with just the naked MacBook Pro -- no USB or other devices
- Unplugged Ethternet and turned off Airport
- Replaced hard disk (which I was planning to do anyway)
- Wiped the hard disk and reinstalled OS from scratch, using migration assistant to bring back my data
- Disabled Spotlight by dragging hard disk icon into Privacy panel
- Ran TechTool. It found some virusy sounding files so I ran Sophos virus scan which found some turds, which I cleaned up
- Removed and reseated RAM

No changes.

Any ideas? I am running out of guesses!!

MacBook Pro, 2.2 GHz Intel Core Duo
4 GB RAM
OS X 10.6.8 with latest software update

Use Console utility to read log files - look for some activity that repeats Many times - at times coinciding with slowdowns.
Run Apple Hardware Test:
Intel-based Macs: Using Apple Hardware Test

Reasons of hanging Mac

Yo have enough memory to support Mac OS X 10.6.8. But, there may be too many stuffs in login items.

Please check and reduce the list.
Make sure you have enough free space on your drive.
Also, get your drive free from unnecessary items like language files, duplicate data useless applications etc to gain some free space on drive.