If it is a hardware issue, my suggestion is a piece of tape on the back of your sim. Trim it to the shape of your card. Do not fold excess over the edges as that will bend out the cage.
Depending on the type of tape (i used electrical)...it should give you enough pressure to keep your simcard pressed to the contacts.

I may have bent my sim removing it from my old Palm...so i had to use this solution. But for a while i could disconnect from the cell network just by turning my N900 upside down.