If you are getting the weaker signals from the north better than the stronger signals from the west, I think it is the trees.
Can you show us a picture of what the antenna "sees" when it is aimed at WCTX?
The WTNH antenna is higher on the tower than the WCTX antenna:
WTNH
Antenna Height Above Average Terrain: 1122' 342m
Antenna Height Above Ground Level: 800' 244m
Antenna Height Above Mean Sea Level: 1443' 440m
WCTX
Antenna Height Above Average Terrain: 987' 301m
Antenna Height Above Ground Level: 620' 189m
Antenna Height Above Mean Sea Level: 1260' 384m
OTA signals constantly vary in strength. I have measured changes of as much as 10 dB in a half an hour.
If the WCTX signal is weak enough to be near the "Digital Cliff" it will be a problem when it is weaker, and you will have pixilation, picture freeze, and finally dropout.
I think you will continue to have the same problem unless you can find a location in the clear for your antenna.
As far as the timing is concerned, that is most likely due to changes in atmospheric conditions, but it could be electrical interference in the evening.
This is what the hill looks like: