主板有问题了

先试一下CLEAR CMOS.

You can try to remove and re-install RAM, although I don't believe that will solve the problem. If BIOS detects RAM error, it will beep when you start your PC. Apparantly that didn't happen. You should NOT try to re-install CPU. It makes absolutely no sense to do that. Plus, every time you re-install CPU, you should re-apply thermal grease. I don't think you have that handy.

It seems like motherboard problem. Even if you find a temp fix today, it will deteriorate down the road. Please return it while you can.

Bottom line: Among the large PC makers, Dell is probably the worst. Dell uses the worst/cheapst parts.