I swapped out an engine in my 92 max with a 91 3.0vg and tranny with 91k on it. When I got on the road and really tested it, it ran fine until I hit 80MPH and then my car started to hesitate and shake and loose power. Then that stopped and now when I get around 50 it does it and if I let off the gas and then accelerate again it does it. The tranny shifts out fine except for the hesitation. Any suggestions. I checked the model number of the tranny before I dropped it in and it's the same as the one I took out! Fluid is good!

 

how is your fuel pump?
fuel filter?
how old are the cap / rotor / wires/ timing belt?
was the engine timed after installation?
is the distributor tight?