I filed a support case with Yamaha on Sunday, and heard back via e-mail today, asking me to call and discuss the issue. I was on the phone for about 45 minutes (mostly waiting) while the support rep also waited for responses from tier 2 support. They asked me some basic questions and suggested some things I had already tried, and in the end suggested I exchange the unit where I bought it. I explained that other buyers were experiencing the same thing, so I hesitated to exchange it for a unit that would likely have the same problem (assuming that if it is a design/manufacturing defect an exchange would probably be for a unit that was from the same lot). They gave me the names of a couple of authorized service centers to visit or ship the unit to. I called one in Newton, Mass which turned out to be a very small area distributor and not a service center and spoke to the rep there. He said that one of his dealers in Connecticut had reported the same problem, but didn't know exactly how he resolved it. However, he suggested I simply use the wire antenna that came with the receiver and see what happened.
Surprisingly enough, I went from receiving 3 stations to receiving about a dozen - most with good HD reception! The auto preset scanning manages to set 7 of them. With manual tuning I can actually pickup a couple of more with very good reception. Adding the supplied AM antenna did initially knock out 107.9, but fiddling with its positioning cleared that up. (I can also now receive WCRB 99.5 in HD which I had not been able to get clearly with my old receiver).
So, I'm satisfied, albeit a bit mystified why the simple wire antenna would work better than the powered Terk. (However, I've long since lost much confidence in Terk as I mentioned before, but still....) I'm sure we've all experienced strange antenna issues before, but this one is the oddest yet.
So, before any of you rush out to exchange your receivers, I suggest trying the cheap supplied antennas that came with the unit.
Happy Thanksgiving !