I have the same problem.

Because the problem can initially be delayed by putting pressure on the sim card slot I have a feeling it's a broken/loose connection on the mother board. There may be a hardware fix similar to the usb connector issues, but no one with sufficient soldering expertise has taken a good look at it as of yet.

On the other hand, I still use it as a wifi internet tablet while my telephony usage is through a simple candy bar phone that doesn't do much more than voice, sms and very limited email.