It might be your new cables between the tender and the engine......
In this video I replaced the cables after the 3v modification, at the bottom of the slope you can see how he stalls and slide forward on his own momentum? I'd used stiffer than normal cables and wrapped them in heat shrink in an effort to protect them, it prevented the coupling flexing properly and made the drive wheels lift clear of the tracks on slopes, definatly worth checking for!
In this video I replaced the cables after the 3v modification, at the bottom of the slope you can see how he stalls and slide forward on his own momentum? I'd used stiffer than normal cables and wrapped them in heat shrink in an effort to protect them, it prevented the coupling flexing properly and made the drive wheels lift clear of the tracks on slopes, definatly worth checking for!