Kill AppleTalk and/or File Sharing, zap PRAM, rebuild desktop, start with minimal control panels and extensions, make sure no apps conflict with power settings (like internet or fax programs), delete preferences files (everything except individual apps, ex. ClarisWorks).

Also, start up Key Caps to see if any keys are active when they shouldn't be, and make sure the trackpad is clean, as sometimes junk can get in there and induce random movements of the cursor, which would keep the machine from sleeping.

If it's not resolved after one or all of those steps, I don't know what to suggest other than a reload of system software.