I had the same problem and noticed that it occurred when one of my doors or trunk was slightly ajar when I locked the car. But this doesn't explain your problem being intermittent unless the door/trunk you are locking itself is ajar.

For keyless entry, here's a DIY thread from awhile back:

http://www.peachparts.com/shopforum/showthread.php?threadid=11777

Good luck..

~Paul

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'91 300E, 208k miles