Ah I see, you reckon this would be causing the low downstream then? Also, is it an easy fix?
Well its either that or congestion but thats unlikely, the fix is to get an engineer out and they should add/remove Attenuators between the cable and the modem to get your power levels into the correct range. If you have a few on there already you could try and see what kind of power levels you get with them/it off.