I had this problem myself, and went through all sorts of troubleshooting steps and multiple tries at each. Reset PRAM, NVRAM, and PMU, deleted power management plists, disabled sleep entirely, etc. I even wiped the hard drive (lost 6 gigs of data I hadn't backed up) and it did this on a clean install with no additional software. Either through some combination of resetting the PRAM, NVRAM, and PMU relentlessly or just because it felt like it, my PowerBook stopped doing it.
I'd say send it in if it won't cost you anything much and let them fix it, but you run the risk of them not being able to reproduce the problem. My PB got really out of hand, sleeping every 5-15 seconds, so I was very relieved when it stopped, even though I lost so much data. Basically, I would make a backup now and maybe try the normal troubleshooting steps. It might go away by itself.