I agree, its a hardware problem. My 13 month old n900 did the same thing (just out of warranty). First it started dropping calls with a connection error every now and then. Within 2 weeks it became unusable as a phone. Initially rebooting would give its IMIE number back, but now it no longer has it when it boots. I tried re-flashing, different sims, cleaning the connectors, basically everything mentioned in this thread. Never overclocked, tried nitdroid on a card, no major drops.