I've had this problem on and off for about a year or so.

Until now I've 'fixed' it by reflashing, but I really didn't feel like going through the bother this time (and it's pretty obvious by now that it's not a software issue).

Instead, I cut a piece of a wide elastic band to roughly the same length as the sim card and put in between the sim and the battery. I figured the elastic would be less likely to shift when the phone gets jostled than paper would.

Seems to be working nicely so far. I'll be sure to report back if/when this fix stops working.