95 shifting problems

Starting from a stopped posistion, I press the accelorator and the van goes but sometimes lags when it shifts. Also, when stopping, the van lags when shifting down. Sometimes, it doesn't shift down so I have to put my foot on the brake and press the accleorator to get it to shift down. If I don't do this, when I press the accelorator the van jerks into gear. I took it to a mechanic, the code came back as unidentified. Is it the speed sensor or something more serious?

I am experiencing some of these same symptoms with my 95/3.8, 147K miles. Most symptoms happen when the car has been driven more that an hour. Most distressful problem is a big clunky downshift into 2nd when decelerating, then accelerating. Second problem seems to be a hesitation to downshift when going uphill, on cruise control, on an interstate. I've replaced the fluid twice on mine, (60K and 120K) Last time I used synthetic, that didn't make a difference. If the problems get worse, I'll change the VSS. If that doesn't work then its off to the shop for a rebuilt transmission.