1. Grinding wheel wobbling
Again a variety of parts can individually or collectively contribute to this issue. If a nut is loose it may have lost its shape and would need replacing. If not, then it would just need to be tightened. The same goes for the collar. Check to see if it is loose and tighten it up if it is.
Finally, the shaft can cause this problem if it is bent. To solve this issue, you just need to get a new shaft and replace the old one.
2. Grinding wheel slips
The adapters and the shaft could have worn out giving too much play to the wheel. You would need to replace these parts to solve the problem. The nut could be loose and if it is, you just need to tighten it up a little. If it is stripped, etc., then you should replace it and make sure to tighten it enough.
The wheel could also cause this problem because it has an out of round bore. When this happens, you will need to replace the wheel.
3. Grinding or squealing sound
The first two sources are the adapters and the bearings. If they are worn out, then they will make noise and need to be replaced. The wheel and its guard could also make these noises.
In those cases you may need to move the guard back so it doesn’t touch anything and check the wheel to see if it is loose. If the wheel can’t be tightened, then you would need to replace it.