|Try with good test ram, good processor "don't assume that yours is good", remove the wireless card, the modem, the LCD. Use an external monitor plugged into the vga out. Make sure the AC adapter is good, a failing adapter can cause all types of issues. |
Also the Make and Model of a laptop can go along way as far as known issues go, so don't forget to leave that information for us.
If I had to take a guess as to what your problem is, I think that you may have a heat issue so check your thermal paste on your CPU and also make sure the thermal pad for your chipset is good and in place.