1988 Lincoln Town Car 1988 Town Car won't going into overdr

1988 Lincoln Town Car

car will kickdown at about 42 miles an hour like it going to shift, then gets stuck and will not up shift. When I put it in the drive position it shifts fine through all 3 speeds, but when placed in overdrive, car does not change like it should. I rebuild the valve body with a shift kit to try and correct the problem but all it did was move the problem up to about 47 miles per hour. Fluid and filter have been change twice the last one hundred miles. Is it the O/D servo that's sticking, are do I just need to buy another tranny. This is the 3rd tranny I have place in this car in the last 3 years.