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That sounds, unfortunately, as if the motor that drives the turntable may be getting weak, the windings failing or the bearing seizing up slightly, perhaps.
Your microwave platter must be checked though, for any friction that might be impeding the turning of the platter, also.
In any case, the microwave would need to be taken apart to some extent at least, whether it is one problem or the other problem.
If it is within 1 year of its purchase date, though, do not disassemble it! That would void the warranty coverage, which would be very valuable in a case of (fairly evident) breakage such as the one you are asking about.
If it is out of warranty, of course, you may take it apart freely - but if you do, be sure to unplug it first and leave it unplugged for at least 2 hours, to allow any voltages of any considerable amount to dissipate, for safety's sake.
Let me know if you need further advice - or need a reference as to where to take it for servicing, if you think it is under warranty or would rather not do the disassembly and repairs on it yourself. Thanks.