Speed control erratic

Same question! Any help, pls!

Same issue here, I sprayed some Electrical Contact Cleaner onto/around the shaft of the rotary encoder, spun the shaft back and forth, and my problem is solved. It's apparently just dirty inside. I wanted to disassemble it completely and clean the insides of the encoder, but I can't figure out how to open the machine. Anyway, perhaps there is no need to get inside. Spraying from the front onto the shaft definitely improved the dial's functionality. It's worth a try on your part.

Common issue on these when they get older, i've stripped apart one of the encoders before and fully cleaned it but that didn't seem to help, you can usually find replacements on eBay quite cheaply. To get to them it's the 4 screws on the casing, you then need to remove the lid plastic then slide the pin out that is holding the lid to the body of the unit, with a bit or persuading the case should slide off and then you can remove the front display to get at the encoder.