"Unfortunately, mysterious computer slowdowns can be difficult to diagnose," so states an article sent to us by Jim Hamm. The writer goes on to explain ten suggestions of areas to check, urging readers to first make sure Mac is backed up properly before proceeding. He goes into specifics and gives useful links. Slow Mac or not, this would be information to check out for your own future reference.
Now De Prez Allen Laudenslager writes to say, "I've been having a problem with my MacBook locking up if left with the screen open and unused for more than a few minutes since I installed a new hard drive last May. I upgraded to Snow Leopard (Christmas present) but that didn't solve the problem.
"After following the instructions in Jim's post and running OnyX the problem seems to have been fixed!"