Often this problem can be cleared up by pressing the keys quickly and repeatedly. If this doesn’t work, you might try getting inside the thing and cleaning any gunk that may have gotten in between the rubber contact strip and the metal contact plates. I’ve also had good luck by simply turning the rubber strip over. It’s a bit difficult to remove and replace the strip, but I’ve done it successfully several times. In extreme cases, the rubber strip may need to be replaced, though I have yet to find a source for an exact match for these strips.