Two weeks ago my Dell XPS 420 refused to boot up. The diagnostic lights on the front panel ( 3 and 4) were lit, indicating that the two memory modules had failed. I removed and reinstalled the modules and got the PC to work for three days, when they failed.
I ordered two new modules and installed them. The PC worked for a week when it again failed to boot and the same diagnostic lights indicated that the memory had failed.
I tested the diagnostic system by removing both modules. Light 1 lit up. That indicated that there was no memory installed.
I suspect there may be a MOBO problem. But if that's true, why would the memory work for a while and then fail? Any suggestions as to the cause?
I've had no previous hardware problems in this 4-year-old PC.
As you already theorized, it appears that your motherboard is failing. It could be a blown capacitor or one of many other possibilities. Getting 4 years out of a PC is quite good. The memory (even your original memory) may not be defective.
You have a couple of options here. You can replace the mobo yourself by finding a replacement (eBay or at a site like http://www.txcesssurplus.com/) or you can upgrade to a more modern computer.
Most Dell's use proprietary motherboards and case connectors, so you can't easily drop in a "normal" motherboard. It also appears that this a BTX format motherboard and those can be a bit more difficult to find.
A search of Google Shopping indicates multiple sources for your motherboard as well. Good luck!