Have you checked the heatsink on the CPU and used fresh thermal transfer grease - overheating might be the cause.
I have a compaq F-700 that im trying to fix for a friend. At first i assumed it was motherboard till i began researching and found this forum. I have tried everything i have seen listed on here. I have taken the motherboard out & heated/reseated the Graphics controller. Same thing the power cycles will not boot up nor will it stay on. The esc key trick will not force it to stay on. The trick with the dvd drive will not keep it on. The power jack is fine and it does this rather plugged in or not battery in or out either way does same thing. When i pulled apart, it seemed to be free of clogs from dirt or dust. I also inspected motherboard for cracks with a magnifying glass. No luck finding one either. Im sure there is an easy explanation for this somewhere. It is running Vista..Model Compaq F-700 (laptop).Can someone please help me with this..Willing to try anything..Thank You In Advance