Ok, so when under load, the engine is misfiring?
Engine runs rough under load?
Has your check engine light turned on?
Ok, thanks. So you have the 3 speed transmission by the looks of it and there are no electrical sensors or computers that control how it shifts.
Based on what you're saying it appears you have an internal problem with the transmission itself. In a case like this I would be looking at the condition of the fluid for starters. Is it nice an bright red or sort of burnt looking. Burnt looking means an internal problem.
I would also drop the pan and inspect for signs of debris in the bottom that will help confirm an internal problem.