Well thanks to everyone's suggestions, I had a major breakthrough yesterday using the Track Signal feature (thanks Steve for that suggestion) to locate a fault in the wiring of my main loop. Despite over 15 years of running MTH trains, I was unaware of this feature until Steve asked about my signal levels. By measuring Track Signal around my main loop, it became quickly clear that in wiring my removalable duck under bridge, I had failed to connect both sides of the bridge, creating a section of track with severe reflection noise from the break in the loop. Fixed that and the signal levels went from unmeasurable up to 6 in that spot to 10s. So it was this dufus wiring mistake rather than any power problems, as the layout now performs well with generator or inverter power.
As a further improvement, after reading the DCS manual again (thanks Vernon for the prompt), I realized I was exceeding the suggested five powered locos max per loop with my engine storage yard area. Will be putting some power switches on that yard to cut down the powered locos in the yard.
Thanks again to everyone's help!
