Desktop

The likely problems are either heat, or the voltage regulators on the motherboard itself.

If this is an older motherboard with DIMM sockets, check that the DIMM voltage jumper is in the correct position.
Some older motherboards support EDO DIMMs, and use a jumper to set the DIMM drive voltage to 5 volts for EDO and 3.3 volts for SDRAM.
For others, there is a setting in BIOS setup for memory drive voltage.

The system power supply can be working fine, but the voltage regulator on the motherboard may have failed.

Make sure that air can flow over the memory modules.
Add a case fan that blows air into the case if needed.

Another issue could be ESD. If you are not properly grounded when handling and installing memory modules, the static electricity that builds up on your skin from just moving around can damage the memory.
ESD damage often does not show up right away.

I use an ESD wrist strap, and connect it to the case before opening a memory package, or removing and installing parts inside the PC.

Chas

This is either overheating , or power transients.
The most likely is overheating.
After the PC has been on for 3 hours, open the case and feel the RAM. It should be no warmer than just barely detectable by feel.
If it is hot, the usual solutions: check for airflow blockages , install fan that blows INTO! the case.
The turbulance, induced by the blades, is important.

Check the ground at the wall plug.
This is done with a voltmeter or a dedicated tester from the hardware store.

Do you have a surge protector installed?
This is an important item. If so, how old is it?
They go bad over time.

The motherboard is not a likely first suspect.

Regards, Chris