The sound output and the rotation of the platter are seperate things. The bad sound could be becasue bad needle/cartridge, bad connections from headshell to tonearm, bad or lose soldering at the circuitboard inside the arm or bad wiring.
The problee seems to me that the speed fluctuates a lot in a short time when playing. This can be because of a blown transistor or a defect in the Main Drive IC. Measure the voltages accoording to the service manual to isolate the component that gives you a problem...
If you don't know what I'm talking about: bring it to a repair shop
