Aha! You really are an Anykey aficionado! Thanks for the input. I think the problem should not be mechanical because if I remap the letter "N" to (for example) the "Y" key, then I get the "NNNNNNNN...." repeat whenever I push the "Y" key. (And the suspect "N" key works fine, as long as I've remapped a different letter to it.)

So I'm reluctant to do the soapy water, but completely baffled as to why these two letters (not keys) are affected.