Clearly the first thing "different" is that the loco will draw more current moving itself, vs spinning rollers.
How about laying the loco upside down in a cradle, applying power, testing (your control test)... then use somthing on the driver like a piece of wood, and apply drag for 10 minutes...
That would prove that more current draw is the issue.
If that does not do it, then start looking elsewhere...
Greg