Multipath is definitely likely with KNWA or KPBI, since your antenna is aimed way off-axis from those transmitters located at Garfield. But with all that ductwork & foil insulation, you're probably getting multipath on more channels.
My TV needs a SNR of around ~15dB or more to keep a lock on a channel. If it dips below that, I get pixelization, and audio dropouts. Every tuner is different, but you should be able figure out about how much signal your tuner cards need, by looking at the SNR whenever you experience dropouts, freezing or pixelization.
With analog, it was easy to see if you had multipath problems. There would be ghosting on the screen that worsened in proportion to how bad the multipath was. With digital, you need a spectrum analyzer. A clean signal would look like this
, and has been compared to the "hair on Bart Simpson's head"... A multipath signal would have high peaks & valleys instead of a consistent "Bart" pattern.