I recently put together a KMAC with vintage blacks.  I found that my 'n' key was doing the same thing.  While typing quickly I was sometimes dropping that letter.  I had to desolder the switch and replace it.  The new switch worked fine.  After I was up and running, I took apart the switch and I couldn't see anything wrong with it.  I concluded that either I had accidentally bent the contact posts while I was swapping the spring or there was just some corrosion on the contacts that was too small for me to see.
Is your board new?  If so look into a replacement.  If not, you could hit the classifieds for one or two red switches and try to replace it yourself.
Edit: year old.  Well I dunno...  Try de-dusting it?