It may sound funny, but first check all the rollers on the turntable roller ring for any 'flat spots'. The 'ring' is either a ring or a star with usually 3 rollers attached on which the turntable rests and rotates.
Flat spot in the rollers can prevent the turntable from initially starting and/or stuttering as it rotates. Unfortunately, if there are flat spots the rollers cannot be individually replaced. You have to purchase the complete ring (or star).
If there are no flat spots then it is most probably the gears in the motor drive which are faulty. The motor assembly is usually very easy to replace and is not expensive.
If you 'Google' 'your model microwave with replace drive motor, in the search box you will most probably get results on how to do it. In most microwaves it generally means removing the bottom plate of the microwave cabinet, disconnecting the wires to the motor, removing the old motor and reversing the procedure with the new.
Safety Notice : make sure that you remove the power from the microwave before opening the cabinet and working on it. Also be aware that there is a large capacitor in the microwave that can give a nasty shock, even with the power disconnected, (perhaps lethal - depending on your individual medical condition) if you touch it.
Microwave ovens rely on the food passing through the microwaves which are bouncing around inside the oven. The food is heated by the atoms in the food being agitated by the microwaves and heating the food by friction. If the food is in the one spot it may not be passing through enough 'wave energy' to heat. It will eventually but how much time have you got.