MSI P6N SLI fps problem

I'm not even sure if this is the right forum to post this in, but I think my problem is caused by MoBo.

Anyways half a year ago I had P4N Diamond (which is top tier in P4N series afaik) and I had PIV 3.6GHz on it - worked fine. Then it got fired, so I got P6N SLI (not platinum, just sli - i think thats the lowest tier in p6n family). When I turned it on, I got a message saying that my cpu isn't compatible (like wtf?) so it dropped the bus speed to 200mhz (800 is standard).

And all the games that I used to run on p4n without problems, now all of a sudden get fps drops. Like I play for 5 minutes fine, then it dropes to like 2-10 fps and i get this slow-as-hell-press-a-button-and-go-for-a-coffee-break effect for next 3-4 minutes. Then it jumps back to 60-100 fps, then back to 2-10 fps... and so on. And I got 2x 7800 GTX's working in SLI.

I got another cpu - Core Duo 6700 2.67 GHz - got no messages of a sort so I figured it works fine. However I still get those damn fps drops.

Since my system didn't complain about cpu, mobo is the only other thing thats different from my first rig. So can this be coaused by mobo? can it be fixed or do i need a new one?