4 db SNR is FAR to low to be stable on 900Mhz. Without knowing the specs on the hardware it's hard to say why it's so low.
Also the 900Mhz Fresnel is very large. I doubt that you are on the very edge of the hill, but more likely your are back a hundred feet or more. It's not uncommon in a setup such as that to get a bounce from the face of the hill that is giving you the elevation, which could cause the symptoms you see.
There is radio modelling software that can calculate this for you. The learning curve is VERY Steep but Radiomobile is one of the best. A Fresnel refraction, AKA a 'Bad bounce', would explain the ridiculous distance measurement too. Lack of TX CCQ is almost as destructive as lack of RX CCQ.