I've had customers that called me with exactly the same problem that was the title of this thread.
And every time, it turned out to be a cheap PSU with a blown 5v line.
I've also had complaints that turned out to be "Bad Ram". Or was it? Actually it turned out to be Dirty Ram.
On the older ram, the output pins on the sides of the chips could get dirt down between them, effectively shorting out the chip.
So before I would throw away a stick of ram that checked BAD, I'd wash it, with soap and water, using an old tooth brush to really scrub the chips and their little pins.
After a good washing, and rinsing in hot water, I'd dry the stick as good as I could and then rinse again with Denatured Alcohol, and dry again.
Finally I'd put the ram back into the motherboard and try it again. In most cases, it would work like NEW again.
I have an old eMachine, that has four RAM sockets on the motherboard. One of those sockets is BAD. Any ram that I plug into that socket will test BAD.
So now, I just don't use that one socket, and I have a little old PC that runs like new, for the first time in years.