Yep - I made sure the modem line is not split after the one where the line comes into the house. It looks like an old cable though, so I may try running a new one and see what happens.
I had a problem with old splitters recently where I upgraded from just Internet to basic TV and, it turned out the local TV tech in my town used 5-900 Mhz splitters instead of the Mediacom approved 5-1000 Mhz splitters. I picked up some splitters at the Des Moines office.