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changing speed turntable

So I finally hooked up my turntable from the early 80s, a Pioneer PL-400 (not high-end by any means, but decent). It worked well w/out issue three years ago when I used again.

Now, however, for about the first 10 minutes or so I use it, it'll run at the normal speed, then, for no apparent reason and at varying intervals, it'll slow for about a half-second then pick back up to normal speed. (This is at 33 1/3 speed).

The only perceptible thing I can see is that there are three small red lights that are always on when the turntable's spinning. When it slows for this half-second, those three lights almost move like they're attached to the turntable. (I'm sure no one has this exact model, but if they did, they'd know exactly what I'm talking about. If you don't, I'm sure you're in the dark).

The weird thing is that after about 10 minutes, this doesn't happen anymore. Odd. Any ideas as to what's going on?