We are running docsis 2 and had a similar problem at an end of line, the modem would come up but drop off at night, the upstream xmit level was like 26 or 30, but I think 41 should be ok. Did you do a ber/mer test on the downstream for the modem? Take a modem at the shop and hook it up make sure its good then revisit the cust. with your known good modem. Good luck.
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