Marathon started struggling to go up a grade

[baymee]

My '92 Marathon, code K2892, serial 716221 has been running great for years. I replaced the batteries about 2 years ago along with all new cables and installed a new F/R switch earlier this year.

Out of nowhere, recently the cart started slowing down going up a slight grade. On the level, it's OK. With the pedal to the metal, it barely makes it up a 5% grade. The battery is fully charged. I looked at the linkage to the speed control and the linkage is all tight.

I have a factory shop manual and the tools to troubleshoot, but this seems to be a popular thread, but none of the answers seem to apply to my situation. The F/R switch worked well for 6 months before this happened.