The car is a 1986 Ford Thunderbird 2.3L Turbo coupe, automatic transmission. Just got it for a few weeks and found it was very jerky when it tries to shift to higher gears, especially around 30 MPH. Took it to a mechanics shop and the guy told me there is a problem on the vacuum modulator so it was replaced. Now the problem is getting worse, it is really hard for it to shift to higher gears from low speed, often the engine has to rev to 4,500 to 5,000 RPM before it finally shifts, at the time the car reaches 35 to 40 MPH.
The problem is better when the car has been driven for a while and got warmed up.
Would appreciate any insight on what the problem could be. Thanks.