GPRSbee network problems after long term use

I have bought close to 100 GPRSbees in the past few years, and most of them are still performing perfectly after several years of transmitting data every 5 minutes. But in the past year, I’ve had about 15 of them stop transmitting, or some that only transmit a couple times a day. At first I thought it was a SIM card problem, but moving the old SIM to a brand new GPRSbee works fine and the new board gets back online instantly. All of our dead GPRSbees are Rev 6b from a few years ago, although I had two Rev 7 boards go bad recently too. I noticed that diode D2 is missing from almost all of the bad boards, but is also missing from boards that are working fine. That diode seems to be installed on only about half of our deployed GPRSbees, so I’m guessing it’s not significant since it only provides backup to the RTC. But is there a reason why the some of our boards seem to die after a few years? The LED blinks fast on powerup and then slow after it successfully connects to the network, but no data gets transferred. We’re using them with our 1284P data logger (similar to the Mbili) and using the GPRSbee library. Thanks.