MTH Alco PA DCC Address Issue

[KSX]

I have three MTH HO Alco PA's (New Haven) running on a Digitrax DCC system. All three ran fine for a short while then none of them would keep an address for very long. Sometimes I have to re-program them several times a night. Sometimes they will not respond the throttle responses and I have to shut track power off to get them to stop. Then they will not restart without re-addressing the engines. Once they were parked on a siding and took off on their own. I have a fairly large layout with eight mains and about 70 engines and all work great except MTH. I have been running DCC for about seven years so I am not exactly a novice. I would hesitate to buy another MTH engine at this point. Does anybody else have similar issues or have any suggestions on what to try?