Find Hidden Problems with Coax Cable in Walls
You might try replacing the splitters.  I had one outside a condo failing in such a way that it caused low frequency signals (&lt;50 MHz) to be severely attenuated but higher frequency signals passed quite well.  The result was that analog U.S. television channels 2 through 4 were garbage, 5 and 6 were snowy, and the cable modem worked very poorly.  In this case, I think the splitters were degraded by flooding (high river levels submerged them for a couple days).