Motor undefined Motor not running consistant, has a miss. Skiping like a car does when not running smoothly.

My experience with electrics has mostly been coffee grinders, hand held food blenders and vacuum cleaners but these are the checks I do. Sadly you'll have to open her up again to do these.
Personally I wouldn't use compressed air. Some compressor pressures can be a bit too powerful. I would suggest a vacuum. Stick the foot end of a pair of tights (pantyhose) over the nozzle that way you get suction but you don't lose nuts and screws up into the vacuum.
Opening her up again might reveal a loose wire you missed rebuilding it. That was my issue with a vacuum cleaner I rebuilt.
-Turn the motor by hand using the shaft the blade bolts to. Your feeling for it smoothly and quietly turning. If it's gritty, noisy or your getting tight spots there's dirt or something inside or a bearing is worn out.
-Test for voltage at every point where a wire ends and connects to something else like switches, circuit boards and the motor itself.
-Disconnect the wires to the motor and test for Continuity. Set your Multi meter to Ohms (the little horse shoe symbol) and touch the test probes, one each motor terminal. You should get a very low number somewhere near zero. Most multimeters will beep when you have continuity. If you get an 'L' or 'OL' there's a short or a broken wire.
-I would then do the same test across all the switches on the machine, wires off and test the switch. Just because they move doesn't mean they're working.
These steps usually work for me so give them a try, the multimeter won't hurt the machine.

Something must be broken if the motor is turning and the planetary is not. It will be one of the following, we will start from the first point of contact with the armature
1) Worm gear broken (fuse gear) you say no teeth broken
2) Worm gear not broken but whole unbroken plastic part not gripping aluminium boss
3) Pin through worm gear broken or missing
4) Pin through centre (or bevel) gear broken or missing
5) Pin through small gear in planetary broken or missing
6) Pin through planetary hub centre shaft broken or missing

The most common ones in order of likelihood are, 1, 2, 3 and 6

Hi,have you set you speed of your rotations,maybe it's set to slow.you have to Adjust the screws in the backof the mixer,so the rotation of the motor is a second apart on the speed setting three.i quite sure that's the right speed number,I have it written down somewhere,if I can find it and it's different il let you know.ty hopethatworks.

burned motor, return to store if covered by warranty, or else have motor rewinded by expert motor technician

Check carbon brushes under the black caps on the motor sides. They only go in ONE way so take care, there is a blocker in the square hole on one corner. Line it up with the missing brush corner. If it's not the brushes remove the back cover & check for broken wires. Do your checking with power unplugged.

I don't know how long you've owned this mixer, but these motors do wear down over time. They don't last nearly as long as the ones made decades ago, the plain basic workhorse mixers with no frills.

Have you tried mixing smaller amounts to see if that works better? Unfortunately with the high cost of servicing, you may find that repair will exceed more than half the value of the unit... leaving the purchase of a new one as your most economical choice.

Open the Transmission Housing and remove the two screws holding
the Motor. Lift it off the gears and test run it by itself. The motor
should run normally at different speeds. If it does, then you can move on
to the gears. Manually turn the Worm Gear and make sure nothing got jammed.
The Bevel Gear should turn smoothly.

The brushes must be insert in a particular manner in order for it to work correctly. If you remove the brushes and shine a flashlight into the hole, you should see a small detent or "stop" in one corner of the hole. Its a square hole. Match this corner with the corner of the brush which is shaved off. The brush is not square; it has one corner shaved or smoothed down. This allows the brush to fully extend into the hole. If the brush is inserted any other way, it will not contact the motor and will not run.

A key is most likley sheared on the center shaft.Take the gears off the center shaft.

The speed control is a governor setup, it's purely on and off, as the weights on the motor shaft spin faster, they press against the contacts, and when they spin a fast enough speed, it turns off current to the motor, which makes it slow down, and current gets turned back on.  That way, you get a pretty consistent speed on a setting no matter whether it's lightly loaded or whipping something fairly heavy.  If you can turn the motor shaft by hand, then you don't have anything binding, but you might have gotten the speed control out of adjustment.  give it another go.