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My Chevrolet Celebrity is having problems shifting into overdrive. It goes to it and drops back to drive. It will shift back and forth until I either mash the gas pedal or I let completely off of the accelerator. It's very annoying and I know it can't be normal. Anyone got any ideas? I had heard that it might be the transmission modulator but I couldn't say.
By the way, this is a 3.8 L motor so I'm not sure of the transmission.  

I started this thread out of hoope that someone will be able to assist me on my tranny problems. The celebrity is a 2.8L 2bbl and it has gotten to the point of not even going into overdrive anymore. Somebody help!!