Thinking cable problem. Check the various connectors.

I've checked & re-checked -- even took the shrink tubes off the end to check for
sliced/shorted wires...

Another thought, although remote. Do you have an older TV antenna with
a built in preamplifier located nearby? One of the manufacturer's
models produces excessive RFI from the preamp. It was the source of
problems with GPS heads located nearby a few years back and people often
forget the TV antennas are usually on all the time.

No TV - but there is a GPS (new) and NavTex antenna nearby... but even with
those things off -- it still happens...

-josh