There's no point just randomly replacing the motherboard. It's just as likely, if not more likely, that it's a RAM or CPU problem. People have just said "it's the motherboard" because they happen to have had that problem themselves but you've told them nothing which should make them think that it's a motherboard issue.
Remove 1 RAM stick at a time and see what happens.
If you're getting the same problem with each individual RAM stick go into the BIOS settings (press F10 or DEL at boot) and undervolt & declock the CPU slightly. If it works with now then the CPU is the problem.
If you can get your PC to start then run a blend test on Prime95 to check if it's a CPU problem and Ramtest to check the RAM.