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?
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