I'd suggest dropping the N900 back to factory settings i.e. no overclock, and then run the N900 healthcheck, and also keep Conky or htop running in the background, and see how it is going. Monitor it over a few days.

If you experience issues, then check Conky to see what apps are running, and using up processor time. The results may be surprising.

I had a couple of days where MM10 and media player were really unreliable, and from conky, I found that the last.fm scrobbling plugin was having issues connecting over 3G and using all of the processor. Oddly after a couple of days it started working normally, and was never an issue on wifi, so I am presuming that there was some issue with the network.

the overclocking could be just on the edge for the various chipsets though. but generally when troubleshooting, the first actions are to ensure all updates available are on, and that the hardware is OK (hence turning the OC off).