Bad mobo is a distinct possibility. But it could also be bad RAM, or even a failing PSU.
Before going any further, just try reseating the memory modules and video card; sometimes this will cure flakiness issues. If that doesn't help, you will need to figure out what the bad component is by process of elimination.
Try testing the RAM in a different system. If you have (or can borrow) a spare PSU, try swapping the PSU. Basically, you will need to either swap known good components into this system, or test components from this system in another system, until you narrow it down.
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