note that there can be multiple reasons for a motor to act like that.
But in my experience. the most common one is that the brush/graphite stick needs to be replaced.
here is a youtube vid showing the proces on a generic brushed-DC-Motor
However it could also be overheating caused by "something else"
No matter what you do need to take it apart to confirm the cause,
but handmixers are usually "pretty simple devices" so it should not be too hard, just remember to take photos so you can put it together again.
Good luck!
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Did you find the cause? I have the same problem.
crwdns2934271:0crwdnd2934271:0 Mario Galvez crwdne2934271:0