Download and run MS's account lockout tools, they can help track down the source of the problem. Lots of stuff can cause it, you also need to check security logs on each DC (assuming you have multiple DCs).
Hardest one I've had to track down turned out be be a disconnected terminal service session, seems although it's disconnected it still polls occassionally and the password is checked so if you leave a disconnected session then change your password you get lockouts after a while.