Would be interesting to know if same behavior in reverse going down the previously uphill grade. Also if you face the engine the other direction does it jerk in forward going down the previously uphill grade. I realize that "other" grade may not be the same 3% but you get the idea of what's being asked.
Do you have a conventional-mode transformer with Whistle/Bell buttons? If so you can operate in conventional with speed-control turned OFF. This simple procedure takes the digital tachometer out of the equation making the engine run proportional to track voltage alone. This engine will speed up and slow down going up/down grades and around curves but does the jerki-ness changes for the same approximate speeds in forward and reverse?
