note that there can be multiple reasons for a motor to act like that. [br]
But in my experience. the most common one is that the brush/graphite stick needs to be replaced.
[link|https://www.youtube.com/watch?v=-pTDF6R9-RA|here is a youtube vid showing the proces on a generic brushed-DC-Motor|new_window=true][br]
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However it could also be overheating caused by "something else"
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No matter what you do need to take it apart to confirm the cause,[br]
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.[br]
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Good luck!