That happens to me occasionally in Vista 64-bit when I accidentally hit the simulation button twice in the editor (Razer Lachesis has sensitive buttons). However I don't get the slow FPS issues, as my rig dominates NoLimits. Its when I go to exit one, the other one freezes up the computer and monitor goes black.
So, I think we have the same issue. I've solved mine by simply taking caution when pressing the simulation button to ensure you dont hit it twice. You don't happen to have a 64-bit version of Vista do you? It seems to be the only thing on my rig that could cause a problem with anything. Someone mentioned RAM.. 6Gb triple channel 1600Mhz overclocked to 1800Mhz + tightened timings Passes 24-hour memtest86 easily. So that is not the issue in my case. CPU + GPU's are perfectly fine as well. It must be a Vista/NoLimits compatibility issue, that's basically all that is left.