The paper in between battery and sim solved for me..
I think it is loose soldering or dry solder on the sim slot as suspected by joerg_rw. in the post here http://talk.maemo.org/showpost.php?p...4&postcount=31

Also, I had this problem consistently after i changed to a new original nokia battery.. maybe the old one had swollen and applied pressure to the sim slot.. The new one created a palpable gap..

As of now i also have tried pulling up the data points on the main board where the GSM antenna connects, Wiping them with alcohol, slightly bending sim slot pins, Only the paper seems to work..