My Legacy trains start up in conventional mode - have to start and stop transform several time to get it to start in Command mode

Have you by any chance changed the order things power up?   It almost sounds as if the TMCC/Legacy signal is not there on first power up. 

David - Thanks No, nothing has changed in the setup 

Might have had the U wire terminal come loose inside the base. Or just broke...

Thanks - I will do a check, I did not think of that that is a great starting point - thank you 

Just in case you missed what David said, he was referring to the startup sequence not your setup.  It is critical that the TMCC, Legacy base and/or TIU are powered up BEFORE applying power to the track.  If they startup up a split second later the locomotives are seeing track power rather than the watchdog signal and will take off.