My JT160 Delta Shopmaster Benchtop Jointer skips. The speed fluctuates.
This sound like a bad commutator in the motor - if you're lucky then it's just the brushes. As the motor turns, the brushes push against metal contacts on the commutator. The brushes provide an electrical path to the commutator. The commutator is a bunch of electro magnets (remember wrapping wire around a nail and putting electricity to it to make a magnet? that's what's on the commutator). So as the motor turns, the brushes connect to different contacts - but if that contact is bad, then no electricity can flow. So the first contact may be good (30 degrees of the motor works fine), but the second contact is bad (30 degrees of the motor turned off), then the third contact is good (30 degrees of the motor back to fine again).
You can try to fix this if you're mechanically inclined and want to take the motor apart. Remove the motor entirely from the jointer. Open both ends of the motor (usually 3 or 4 really long screws going through the motor to join the ends). The brushes are usually spring loaded and can shoot out, so go slowly and look inside while you open it up. The brushes should be at least 1/2 inch long (if they are shorter than that then replace them). Also look for sawdust - if your motor is jambed full of sawdust you may just need to clean it out.
Once you have the motor fully apart - the commutator is that bulk of the shaft, with lots of wire wrapped around it, and one end having metal contacts (where the brushes ride). Examine the contacts closely - if one is bad (broken or missing) then you HAVE to get a new motor - no way around it. However, when they put the contacts onto the shaft, they use lots of epoxy/glue. This is what is between the contacts (from side to side). As the brushes move around on the contacts, they also ride on the glue - this can "smear" the glue from between the contacts to actually cover the contacts. If this is the case you can use 600 grit sandpaper to clean the contacts. I would also cut the glue between the contacts so it sits lower than the contact surface (take an old hack saw blade and brake it in half - the width of a hack saw blade is usually the same width as the distance between the contacts - lightly saw the glue between the contacts until it's lower than the contact surface).
While the motor is apart, check the bearings - if the bearing are bad, the shaft can "flop" back and forth and might make the brushes skip over a contact. If it's a bearing you can usually get a number off of the side of the bearing and find new ones (search for the part number on google - bearing are about $2-$4).
I'd also check the belt the goes from the motor to the jointer shaft - I like those powertwist belts because they don't deform and induce vibrations. Read More At : JT160 Shopmaster 10 Amp 6 Inch Benchtop Jointer...