If you have a 25+ year-old Technics belt drive turntable, and it won't maintain a steady speed, replace the belt.
If that doesn't work, probably the speed controls themselves are ruined. These "trimmers' or "potentiometers" are composed of carbon, and deteriorate over time. There are two of these devices, located on the underside of the turntable.
First, remove the headshell and put it in a safe place. You don't want to spend the time to repair the turntable, only to have destroyed your stylus in the process! Then lock down the tonearm and remove the platter.
Remove the bottom cardboard cover, and locate two circuit boards containing the "pots".
The "coarse adjustment" board is here. One is for 33 RPM, the other for 45:
Here is the other; the Pitch Control:
If you feel lucky, you can give both of these a shot of WD-40, and work them back and forth. Sometimes that fish oil does the trick, at least temporarily.
For a permanent repair, purchase the following equivalent parts. Mouser Electronics is a Web source, and these are their part numbers:
coarse: 652-3309P-3-203 (need two if replacing 45RPM)
pitch control: 531-PC16SH-4.7K
The pitch control pot is held in place with barbs moulded into the body:
Trim these off, unsolder the bad pot, and enlarge the metal bracket to accomodate the new one.
The new one is a little longer than the old one; trim the shaft.
The new pot is held in place with a washer and nut, supplied with the device.
Replace one or both of the coarse adjustment pots. Set the value with a small screwdriver to the middle of the range of adjustment.
Reattach the pitch control knob. You may need to enlarge the hole on the knob, or trim the shaft. A little hot-melt or epoxy may be required. Ensure that the knob turns freely without rubbing the case before reattaching the bottom cover.