toledodelot - same issues as you. I'm on an Olevia TV, on the internal tuner, it fund 378 digital channels and 74 analog channels. Instead of getting the channels on the listings they give, I get a corrupted signal on the actual broadcast locations (11.1, 13.1, 13.2, 24.1, 24.2, 24.3, 36.1, etc). The video macro blocks and drops/garbles audio. The odd thing is I get their PPV promo channels perfectly in HD. Haven't done any research yet to figure out what is going on. Want to be sure my TV should actually be able to get the channels off cable before I go fussing.
Meantime, the OTA signals are nice and strong and crisp and clean. All the OTA signals are good picture quality and reliable for me, be it Fox, CBS, ABC or NBC.
Edit - supposedly my TV has an "unsupported" QAM tuner - which translates to the mfr only designed it to get the ATSC or NTSC signals, but it might get some cable signals. Oh well. it was cheap.