Intermittent keypad entry problem on 98 Explorer

I just started having an intermittent problem on my 98 Explorer numeric key pad
entry system. The symptom is that I enter the keypad code sequence and I can
hear the lock make a noise and the lock "button" goes up about 1/4" but then
back down. I use the remote and it opens up fine, and after that they key code
works fine. The first time it happened was a few weeks ago. It happened again
the other day and I tried the key code 5 times with the same result. I then
used the remote and it opened right away, and using they keypad seems to be OK
again. Any ideas?

Thanks.

Ben