Mine does it too, started soon after I dropped it months ago.

Does it every few days or so, several times, and it seems to help when I hit it, or try to bend it, or something. Big pain in the *++(9!3 when it happens.

I've tried a new sim card, cleaning it, bending the sim cage to push it harder/tighter into the contacts, new battery, and even tried a little trick I saw somewhere on here to place a piece of paper or cloth inbetween the battery and the body (seperates the battery from sim more???). Just tried that 2 days ago, and I thought it was good, but it did it again today.

Nothing has really helped. I'll probably send it in to get repaired, that plus the FM transmitter, plus the potential usb port (has acted up a couple times, but nothing definite yet)