Melex 412 slow up hills
 

I have a melex 412 with a new motor, new batteries, new cables. Rolls freely when pushed. I know it’s old but it works great on a flat surface. When it is on a incline or thick grass it just slows down I have a digital battery gage on it and starts going down, but when you let off the pedal battery numbers come back up. Very sluggish when it is like that on a hill. Motor is not hot. I don’t know much about them and would love any thoughts and ideas what I could check. Thank you so much.