1990 Eighty Eight Transmission Shift

I have a 1990 Eighty Eight Royale Sometimes while accelerating, the transmission will stick in a lower gear and refuse to shift; once I let off and slow down for a stop light it goes back to normal. Most of the time it shifts fine, I've never had it happen accelerating to higher speed onto the interstate. Fluid level is ok, car only has 70,000. I have seen other postings referring to problems with the Vacuum Modulator. Could this be the problem or are these symptoms of a more serious problem. Any opinions would be appreciated.

Thanks

Vacuum modulater does sound like a likely culprit...unless the fluid hasn't been changed. The fluid and filter should be changed every 40-50k miles. When the fluid is changed I very highly suggest that Lucas trans-slip is used in place of a quart (approximately) of dextron mercon.

Vacuum modulater does sound like a likely culprit...unless the fluid hasn't been changed. The fluid and filter should be changed every 40-50k miles. When the fluid is changed I very highly suggest that Lucas trans-slip is used in place of a quart (approximately) of dextron mercon.
Thanks for the reply....I'm not sure if the transmission fluid has ever been changed in this car. Will the transmission oil pan need to be removed to drain the fluid...? I've never changed transmission fluid before.

Yes, to drain the tranny fluid you need to remove the pan. Just loosen the bolts on the narrow end of the pan first and work your way back to cause the pan to angle as it drains. Another bit of fluid is in the trans lines to the radiator so you may want to disconnect those to get a bit more out.

Add the lucas trans slip and dextron mercon 3 till it reads full (3-5 bottles on mine). Then drive it a very little bit and check it again and refill it to full. Just don't overfill it or you start to run into the issues caused by the added pressure of that extra bit of liquid.

Filter/gasket, lucas trans slip, and tranny fluid should run around $50.

Why not try a new 15$ Vacuum Controled Tranmsission Modulator first? Usually more simple than changing a tire.

Why not try a new 15$ Vacuum Controled Tranmsission Modulator first? Usually more simple than changing a tire.
I did Change the Vacuum Modulator....It was cheap and easy. The transmission does seems to shift smoother, unfortunately I still have the shifting issue. I have yet to change the transmission fluid / add the lucas trans slip. I will do this next..

Now that I've had more time to evaluate the situation, it has only happened when the car was wasn't warmed up all the way....within the first 5 minutes of driving. Not sure if this gives any more clues.