manufacturer specs are +55 on the return. Anything over, meaning +57 would knock the modem offline even if your downstream was good. Bad power cords on the modems can cause lack of power for transmitting, bad connectors can affect this as well. Just as bad splitters, bad cable, bad tap plates and the HFC plant. Just some advice.
This is CABL.com posting #147778. Tiny Link: cabl.co/mMBG