Perhaps the metal contacts are bent? Also, see if your carrier can replace your SIM card, i've seen some old cards that had quite a dent on each of the metal pads from the years against the little springed metal prongs the mobile had in the interface.
could just be a faulty sim card. If I recall had this issue losing signal and intermittent sim card error.
spoke with my network provider and explained to them that if I put my sim card in another phone it works fine, they basically said that I had my sim card for about 2 years and it was the most likely reason and it was!!
