bobnielsen· Éminence grise
It sounds like either be a bad multiplexer and/or dish or there is a cabling problem (are the cables quite long or do they have copper-coated steel conductors?) Diplexers should not be used with the 5LNB dish or WB68, but I can't see where they would cause those problems. I suggest connecting 4 of the tuners directly to the dish, bypassing the WB68. If the problem remains, it is probably a bad switch on the dish itself.