Trainboy4449
Hello & Welcome to the Forum!
You must have a pretty big Christmas tree to run a Big Boy around it...

Common problem with low power available using an analog transformer on a tmcc train.
These digital systems are made to receive full power to the track constantly. When using the lower voltage you approach stall speed(jerking, erratic action, etc) durring start up or low speed operation..
so.......
go back to using original tmcc power system, continue to use as is(make sure switch is in conventional operation position), or tear out the digital electronics and convert it to an analog  operation train....

Happy Rails!
dave