It is possibly software, possibly a hardware fault, possibly an issue with your SIM card.

First step you should try and reinstall the firmware I guess... I've not done that myself so I'm not sure but I'd imagine there's an option in the updates manager.

If that fails, get T-Mobile to send you a new SIM card (not so likely to be this, but just in case).

After that I'd say send it back for a replacemnt really... I've never experienced anything remotely like you're describing and assuming the above two options fail and it is not listed as a bug in the Maemo bugzilla (ie. a genuine know issue) then it seems like a hardware sort of issue.

I hope that helps and is not just stating the obvious!