Did you alrady flash it succesfully? (EMMC also) The latest firmware is probably the way to go, IMHO the stabelest thusfar.

Did you check the outsides for dents and scratches? The build quality (don't get me started about USB port however) of the N900 is pretty good, and if overclocking could have harmed your device you'll probably have more issues performing heavy tasks, but that locking could be a hardware problem. Try locking it via power key and then lock screen, to rule it out.

Where do you live, and what about warranty? Knowing that the n900 came out less than 2 years ago and many carriers offer a 2 year warranty, (or it was nkia factory warranty, not sure) that should be worth a try if it cannot be resolved by software.

Come on guys, its a N900 he got. Theres a good chance he made right deal if there is no major HW flaws no?