What I used in my HE was Digital Stream set top boxes. I was not sending out a HD/widescreen signal on the all analog system, but I let the box reformat the signal to 480p/4:3 output. They work beautifully. Initially (this was long before the NTSC/ATSC switch was actually made) I too had issues with dropouts. Multipath can be caused by something as simple as a tree limb depending on the antenna type. With ATSC I found that cut antennas were actually horrible when multipath was an issue. I went to a high quality consumer grade antenna and had no issues since.
People seem to like to make things more difficult/expensive than it needs to be. I like/follow the KISS theory; Keep It Simple Stupid and it has always worked for me. I've joined antennas that people tell me can't be done and work, and they work.
Anyway, try a regular set top box and a modulator and see what happens if you can. If you still get dropouts then you have a reception issue and not an equipment issue. And to be honest, I think reception is your problem because I find it very difficult to believe that every single demodulator is defective. Also make sure you are not overdriving the tuner with a pre-amp. Good luck.
