RM 830 e runs rough at high throttle

[leveled]

Hi I have had this tractor for couple years and have had it for backup so it is not used much. It runs alright except at high throttle with a load on the engine like cutting taller grass or going up a hill it kinda throttles up and throttles down like its misfiring or losing fuel or spark. so every once in a while I will play around with it and try to get it working right. I have cleaned the carb checked and changed the fuel, checked the spark plug, set the gapping to the flywheel to ignition mojigger, checked for intermittent spark. I once took the tractor in a had a local shop rebuild the carb. still no avail. (think they ripped me of) They actually told me this was normal. Kind of lost on what to do next any suggestions are welcome. It has a 8 HP BS .Thanks to anyone reading.