Thanks, giqcass...
Jack, you now have, what, 3 separate threads going on this? If I follow what you are saying, you have now replaced your equipment (cable modem and router) and you still have the jitter problem. If so, then you need to resolve this with your cable provider. This is a common issue with cable internet, and it's a PITA to get them to fix it. It's usually caused by a degraded or damaged electrical connection in their coax cable runs. For example, a section of coax that has moisture seeping into it, torn/crushed/rotten insulation or dielectric, or corroded or lose connectors. Demand that they send someone out to find the problem.