Instantaneous shutdown of my Toshiba Satallite A105 Laptop

I have a toshiba Satallite A105 laptop, it shuts down instantaneouslywithout any warning or sign. I gave it to some repairshop thinking itwould be the overheating problem, they said that they cleared up thedust and everything. Now the fan seems to run fine, I can hear it! butthe problem came back after 4 days.. fan is still running fine! Don'tknow what is the problem now, sometimes it runs absolutely fine for30-45 mins before shut down and sometimes it doesnt even boot properlyand shuts down. Please suggest what could be the problem. Also, Ithought may be it's my battery which is causing the problem cuz it;sworn out totally, so i tried running it by removing the battery anddirect with the power source, but even that didnt help.. same issue!:(