I'm not experienced enough with turntable hardware to know whether this is a common problem or not. It would appear that while I'm playing records, every now and then, the music seems to dip maybe by a semitone. Is this a common thing that can happen to record players over time? This is a Pioneer PL-300. What causes this issue? I'm pretty certain it's not the record.
I've experienced something very similar with my turntable setup.
I have a Pioneer PL-4, which I'm assuming is about the same era as yours (mid-to-late 80's), and while its speed is rock-solid at 33.3 RPM, it's all over the map at 45.
If there's a speed adjustment knob, the potentiometer could be dirty. Take a look at this post[/u] (http://www.hydrogenaudio.org/forums/index.php?showtopic=86342&st=0&p=740731&#entry740731). Or the motor shaft might just need lubricating (if you can get to it).
If it's a mechanical problem (bearings or motor, etc.) it's probably impossible to get parts.