I had a similar problem and it turns out that I had a bad LNB. There are 2 different gains on a LNB, one is 13 and the other is 18. Don't know which one, 13 or 18 was bad but, had to switch the LNB twice. The first LNB came out of the box bad, and the second LNB worked like a charm. All signals up to strength in the high 90's on all TPs and channels. You get about 18-24 months out of a LNB depending on your local weather conditions. I suggest you first check your Disc Switch, then your line runs from the dish, then your LNBs. Sounds like a LNB problem to me.
