Good experiment. (Just don't expect Internet connectivity or your PVR to be recording anything whilst ongoing.)you can try one more experiment to try to track this down. At the demarc splitter, disconnect your upstairs coax, connect a barrel connector (F/F union) , and then disconnect your downstairs coax and connect it to the other end of the barrel connector. see if the modems sync. If they do, then it should indicate an issue upstream.
Honestly, it's tough to have a simpler setup than what you have, short of a direct coax connection, so one has to begin wondering if some cable or termination, or adapter, is faulty.