This sounds like the CPU is overheating. Dells are horrible in this way: The heatsink tends to draw in a lot of dust. Opening the case and fiddling around could have knocked some dust in the air, only to be sucked in by the fan. Try this: Open the case back up. Compressed air is recommended, otherwise you'll need to use your own lungs for this. Blow/blast as much dust from all the fans inside as possible, especially the one above the processor, and reboot.