Presario 5070 weird behavior

I have a Presario 5070 that is about 4 years old and it has started doing
some weird things. I have already gotten a new puter but I would like to try
to fix the old one if possible. Most of the time when I hit the power on
switch it does nothing, won't boot up. But after I pull the AC cable out of
the rear and put it back in again, it will boot up. I tried replacing the
cable, no difference. The clock also loses time even though I replaced the
battery on the motherboard. Any ideas what could cause this behavior? Mike N