Marx 999 problem

I have a Marx 999 with the early prewar motor. It runs good, but the problem is I can start to run it and after a few minutes, it slows down like I moved the throttle lever. A while later it will take off either normal or even faster. I have tried several different engines and this is the only one that does it. I'm powering it with a Lionel zw275r. What would cause it to slow down and speed up? I don't think the zw transformer is at fault, none of the other engines are affected like this.

Thanks