said by Sirion8:
I swapped my old Motorola CyberSURFR modem for a new Toshiba PCX2500, problem solved!
Just so you know, the old CyberSurfr modems used a proprietary non-DOCSIS system, operating on a completely different channel than your new DOCSIS Toshiba. So in your case, switching out the modem resulted in far bigger changes than your average swap of one DOCSIS modem for another. I'm not saying that modem swaps don't sometimes solve problems, but you actually underwent more than just a simple modem swap.