That sounds similar to the problems I was having and I resolved them by removing a splitter I had in line before the Cablecards (I had 2 splitters before and now I only have one). It sure sounds like a signal strength issue to me.
Velocity Micro Cinemagix Grand Theater, C2Q, 2GB, 2 TB, 2 DCT, 2 HDHomeRun
XPS420, C2Q, 3GB, 1DCT, 2 HDHR
Homebuilt C2Q, 2 GB, 1.5 TB, HVR-1600, FireSTB
Dell E1705, T7600, 2GB, 250GB, HVR-950
WHS, WRT600N, DMA-2100, DMA-2200, XBox360