Possibilities:
1) You are diplexing. Diplexing won't work as before, and may not work at all in some installations. D* suggests all diplexers be removed.
2) You have one or more faulty BBC -- swap both of the ones on the "bad" HR20 with those on the good one. The problem might move, or might be "fixed." In either case ask D* for two more BBCs, claiming the problem moved.
3) You have a faulty HR20. Try swapping it with the good one to see if it's the unit or the cabling.
4) You have a multiswitch and it's got a problem. Note that only the Zinwell WB68 or WB616 is supported by D* with the new sat. If you have something else, or if you have a complex setup with cascades, you may have to fix this.
5) You have a problem tuning the LNB on the dish. This might be in the HR20, in the multiswitch or in the dish itself. In theory it could be in the cable or the (lack of) grounding. Is the dish grounded? It should have a grounding nut on it someplace.
Problems 3 -5 can be sometimes fixed with a Zinwell WB616, which has its own power. I had a #5 problem once, fixed by swapping out the dish. Bad LNB.
Intermittents are tough, so narrow it down as much as you can by swapping before you call D* for help.