Corrupt Xilinx 18vxx poms

I'm using a Xilinx 18V04 prom and find that it occasionaly loses it's program. I can program the prom without a problem. I use it for months, and then one dy when I turn on the board, some of the bits on the prom are incorrect. If I reprogram the prom everything works normally again. This has happened on several different cards. Has anyone else seen this problem. I searched through the archives on this newsgroup, and found someone with the exact same problem, but no solution was offered. I contacted xilinx, and they have one of the parts that failed, but claim nothing is wrong with it. Any information would be much appreciated.