Even though the drop is new/replaced it can still be bad (damaged during bury, I know, I've done it more than once!) Run a test, hook the modem direct to the tap with a new cable. That will once and for all eliminate the drop as a potential issue, if you haven't already done so. I would still by pass the amp for the modem, -6 or so should be fine with a modem. JMO. GL
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