Same problem , started in the end of April.
Was running older Maemo release when problems started.
Upgraded to latest rel but issue remained.

Phone was two weeks in repair and they could not replicate the issue. Phone was flashed there.

Tried different SIM cards but issue still remains.

Today gave phone to alternative repair shop.
Interestingly customer right before me had exactly the same issue with n900.

Only thing i noticed during SIM problem are acwake events in dmesg. Not sure what they stand for.
[42212.271911] McSAAB: ACWAKE DOWN
[42212.271972] McSAAB: ACWAKE UP
[42212.272033] McSAAB: WAKELINES TEST OK