I'm amazed at the range of audio quality. There is a constantly variable quality noticed here with Direct TV.

ProLogic II should not solve what you are hearing. It’s not a ‘clean up of loose bits’ It is simply a steering mechanism, which redirects information. As are all other DSP modes and each should be able to direct the sound without bleed over or any sort of aborted attempts to whichever speakers they are steering (“seems to come out of the mains but cuts in and out”).

I have just recently had trouble with the stream being fed from Direct, but with HD channels where sound can warble a sec, and dropout. This week usually it’s just gone during several hours at any given time on HDNET. The rest of the SD channels this should not occur, unless your seeing video dropout at the same time when weathers coming in etc.

I would say check all connections and most particularly check your ground for your Satellite, it should be grounded to the main house ground and not separately (as some installers attempt). Check all connections from the exterior in and your interconnects. When the signal is more compressed (as some channels are),
I’ve found any borderline interference will become more apparent. The sound your describing sounds like an issue with interference, which is sporadic. Ex: you could find that this situation occurs due to a speaker cable picking up electrical from the power cord being situated too close when a large appliance kicks in. etc. Check the settings in your Direct receiver, some settings in my HD version of a box will work perfectly with some channels , - yet not others, - creating audio/video artifacts if selected incorrectly.

What your describing most certainly seems connected with the handling of the DSP modes, (you mention avoiding it by using stereo). And could be, you will, - trace it to a faulty part in the 1050, but this has the sound of an issue elsewhere in your processing of the stream.