based on the fact you are using a preamp, as you posted in this other thread, and are only 3.4 miles from transmitters per your tvfool, the symptoms you describe are implying you are having intermod distortion problems, not multipath problems.
correct me if I am wrong here is your tvfool:
4 bay antenna question
Lowering the height, adds more pathloss between your antenna and the transmitters,
ie obstructions like your neighbors houses, etc. reduces the strength overall of signals entering your preamp, which in turn gets it out of intermod trouble, which in turn reduces the noise floor, which results in better reception overall. As described by holl_ands numerous times, if your preamp is in intermod territory, for every 1 dB of reduced input signal, reduces the output noise floor by 3 dB.
Try the same test without the preamp at all, and I suspect you will have much different results. You might have better luck keeping the noise floor under control, using an antenna with only modest gain. Say a GH without any reflectors or even 'stealth hawk'.
I would really suggest getting a PC based tuner that can indicate both Signal Strength, in dBm and SNR in dB, so you will have a better understanding of what is going on.
Many devices like the hauppauge 950q, or the hdhomerun are not that good at accurately displaying the 'strength' of signals to the user, because they "Top Out" too early to be useful for troubleshooting like this. My favorite device for such tests is the newer hauppauge 955Q.
Which seems to have a wide range of SNR indication, and also provides an indication of Strength in dBm.
At least when used in linux it does. I can't speak for windows, since I don't use windows.