Old Mixmasters need to be serviced!
Usually all you have to do is give these a good service, and they will run just fine again. The grease and oil get gummed up and cause the Mixmaster to bind and run hot.The old grease turns to a sort of putty in the gearbox. Tear it down, remove the old grease from the gearbox,pull the shafts and gears. Clean gunk from the shaft bushings, regrease (I use automotive wheel bearing grease) clean everything up, and use a good grade of machine oil (I use 3-In-One) on the bearings. I run the mixer for quite awhile while oiling it. Usually it will "burp" up some nasty looking brown shit out of the oil holes. Wipe it off, and give it a few more drops in every bearing, including the top of the shafts. The Mixmaster will start to run faster and stronger as you keep lubing it. Finish up, and give the sucker a nice coat of auto wax and your all done. Takes a few hours, but the mixer will be serviced as it should have been years ago. Now that you have done this service, the Mixmaster should only need a few drops of oil at the bearing holes, about once per year.
Here is a pix of a couple model 7's I restored last year.
It just happens, that I restored a model 9 the same way this afternoon. Just finished putting it back together just now. I have to get a new cord for it tomorrow, and repaint the black base, but the mixer is running like new again. I don't think the model 9 was ever lubricated. It would barely even run at all.