Small Engines (Lawn Mowers, etc.): snapper lawntractor model#331413kve serial #35270168, rear axel, small spring

Question
This tractor has big problem going up any kind of incline.we changed the tension on the spring changed brake spring adjustment replaced the small drive wheel on rear axel and the thin friction paper it was alot better for short period of time then went back to the same however it goes good in reverse. when we go back to the cottage it seems to go alright for awhile you would almost think it had something to do with overheating and then slipping. iam at aloss on this one thank you in advance for any help you can me  

Answer
there should also be a small spring from the disk drive holder ,  by the disk  drive unit.  mounted to  tractor frame at some point.  as you sep on peddle it pulls disk drive wheel  which is a black rubber part off of disk, to change gears.  onec peddle is released it aplies it back to drive with some pressure.   note if this drive unit gets any kind of wet????   it will slip some for awhile til it is dry again.