Cyclone II Serial Config

I'm having a rather odd problem and I was wondering whether anyone else has experienced it.

I'm using an M25P40 serial prom with an EP2C20. Standard setup, in fact we reused the schematics from a previous project and repinned the FPGA. The only difference is that it's a Cyclone II instead of a Cyclone part. We've only built 4 boards and they are all flakey.

The problem is that the proms are mysteriously unprogramming themselves during normal operation. That is, the Cyclone II won't configure until the prom is reprogrammed. Sometimes the serial prom can't be reprogrammed and must be replaced.

Any ideas?

Thanks for your help.

Leslie