Sudden, strange Accuphase FM tuner behavior

I listen to a lot of FM radio through my Accuphase T-100 tuner which has been flawless since I bought it about six years ago. The signal level, as indicated by the signal meter on the front of the tuner, is usually just above 4 while listening to a local station (the meter goes from 0-5). While listening the other day, the music stopped and the signal level went to 1.5. I discovered that if I put the FM muting selector to “1” or “off” instead of “2” (where it usually is) the music comes back but with a little more static than prior to this problem. The signal level is still at 1.5 with the FM muting selector at "1" or "off". What is going on here? The service manual is available here, but I couldn’t find anything that points to the cause:

http://vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Accuphase-T-100-Service-Manual.pdf