What type of LNB is it?
What are the settings in the Receiver for it?
If these are all ok and you say you swapped out the LNB and have the same problem then swap out the 110 LNB with the 91 LNB and see if the problem moves. ( Bad, new LNB YES I have had that problem)
If this is still happening then do a factory reset of the STB and see if it still happens.
If all that fails borrow a STB from someone and test it out. Either that buy another STB as a spare (you should have two anyway).
If the new STB works than guess what?