Random reboots topic

My SP reboots RANDOMLY and it said SOMETHING about SIM CARD REMOVED before it reboots... i use it with this sim card for 8 months... why this happen now ? what can i do ? i was on STOCK STOCK rom rooted, and i go to existenz rom to see if the rom was the problem... but now i just see that its something like SIM CARD REMOVED & reboots... help please