What causes a 1995 buick century front wheel drive transmission want to up and down shift at 57 miles per hour?

There are four things that control your automatic transmission shifting. First: Speed sensor Second: Throttle Position sensor Third: Automatic Transmission Modulator. Fourth: Viscosity (How good) your transmission fluid is. One of those three are probably bad, first start with replacing the fluid and filter in your transmission (Take to a professional for best results), second,look up voltage specs for the Throttle position sensor, test it (It's on your intake ductwork) If it's not within spec, you'll have to adjust it. If it's fine, replace your speed sensor (Easier than the modulator, and it can still register correct speed and send off a bad signal to PCM, causing incorrect shifting) and lastly, replace the modulator. That should fix your problem