I have an emachines 220 and it keeps switching

I have an emachines 220 and it keeps switching itself off at first it did it occasionally and the more the machine was on the more frequently it started to do this. I suspected this may be due to overheating and put it into a pc repair shop. Memory in the pc was low so extra was added, since it hadnt been connected to the internet in some time lots of crucial updates were done. The man in the pc shop checked the processor and fans and there is no sign of a prob with overheating. After putting extra mem in and cleaning the machine up and installing windows updates it wasnt switching itself off and after getting it back from the shop it ran fine for a week then started doing the same thing again although not as bad as before. It even has switched itself on since I got it back. I installed things to check HDD temp and fans and it all seems ok sometimes the pc switches off just as soon as its been turned on, sometimes it stays on for hours before doing it, occasionally the lights on the front suggest its hibernating but when i try to get it to come on and move the mouse or touch the keyboard it doesnt come back on. Does it need a new motherboard, what could be the problem? Motherboard is IM845GV Emachines 220 (2003) Intel pentium 4 processor. I am not great wi computer hardware but any help would be gratefully appreciated. Thanks