SD9 Sound Unit Stall Issue Resolved

As we were running some test trains over the layout for the first time in months, we noticed that the two SD9s were acting up again.... but finally a new clue arose to help pursue the cause.  They had been overly sensitive to power drop-out, making the decoders reset with the slightest power interruption.  Now these units were resetting when an adjacent power district went into overload mode.  In other words, if a train in a different sub district on the PM42 shorted out, the PM42 would cycle its overload relays, which caused a power drop or sag in the sub district that the SD9s were in.  Unlike any other unit on the layout utilizing the same decoder board, this demonstrated that these decoders were not fixed as previously hoped.  After trying an experiment with super-caps to make a temporary keep-alive circuit (as Ray has been installing in some other HO units), we did some extensive testing and were able to determine that the sound level output on these two units, had a direct relationship to the problem; but no matter what the sound settings, the units would only work reliably as long as they still had an oversize bank of capacitors attached.  After messing around long enough, I finally got to the point where I just cleared out the custom 567 sound project that I had previously installed, then loaded the Digitrax GP38 file back into the decoder instead.  Problem solved!  It appears that it was a corrupt sound file causing the stall issue, or an unknown sound CV setting?  Lot of lost hours in getting these sound units back in reliable running order, but glad to know operators will be able to run the local jobs from now on without issue... even if they don't have the correct sound project for now.