Why is my HTC Tilt not recognizing my SIM card?

I have an HTC Tilt with At&t since Feb. and it works fine for a period of time, but after a while something seems to happen and when I look at it to check time or something it shows me,"Please insert SIM Card." At first I thought my phone was locked but it works without any problems half of the time so it can't be locked only at times... If anyone out there knows what I can do I would really appreciate it. I have checked different forums and i still haven't found the right answer, i have also called At&t and they have not being able to give me a solution as well as HTC. When i have this problem it goes out on me for about 1-3 days and i just have to keep trying on and off to see which time it will turn on without a problem. I have a BlackJack which was my phone before that, which i never got rid of so when this happens I switch my SIM card and it doesn't give me any issues on my blackjack. I really love my HTC Tilt and would love to be able to use it without carrying my BlackJack.