Replaced motherboard, laptop now shuts off after a few minutes.

I have a Compaq Presario CQ61 that would not POST or boot. I replaced the mobo and the laptop POSTs and boots into Windows, but after a couple minutes it kicks off. It does this in the BIOS as well. The fan comes on and stays on then kicks into high gear leading me to believe it's a heat issue. But the fan works, I applied a liberal amount of thermal paste, so idk what is doing on. I removed the drives, RAM and external cards to no avail. Any ideas?

 

Enter bios,go to health status and check your cpu temperature its the first thing you need to check after a rebuild,keep your eye on it until it shuts off and post the numbers and your operating system.

Make sure your power supply is functioning,this could also be caused by a bad power supply,does the power light light up when power is connected?If you own a multimeter your can check the voltage matches what's written on the side,you can also try removing your battery and running just on AC just in case the battery has a serious fault.

If the cmos battery can be accessed remove it for 20 seconds then replace with the main battery and power disconnected to make sure the bios has been reset if not already done,I would try this last if it requires disassembly.