passenger door keyless entry problem

My 94 Camaro Z28 has developed a strange problem with the stock keyless entry system. It is very flaky, working sometimes and then not working later. When I press the keyless remote twice to unlock the passenger door, sometimes it will not unlock. I have checked the door lock itself, it is not binding or jamming and works fine manually or with the lock/unlock switches inside. The drivers side always works fine with the remote. This has just started recently, I have never had this problem before a few weeks ago. I have changed the batteries in the remote, but this did not help. However, the passenger side will lock fine with the remote, it just doesn't always unlock. Anything else that I might look at besides the keyless entry module? I can't think of anything else, but maybe someone has had this problem before and has a simpler and cheaper fix.