I have 1997 Bonneville with 101k miles on it. I had the transmission rebuilt about 8k miles ago. When the car is warmed up, usually takes about
30 minutes, the car will surge in about 200-400 rpm intervals at highway speeds. It is especially bad on loads or inclines. It will eventually do this at lower speeds too, after I have been driving longer. The rebuilt transmission has been replaced by the transmission shop and it still does the same thing. When you bypass the computer and manually lock it up, it will not have the problem. All of the wires to and from the transmission have been checked, as well as all of the grounds. Also, no error codes are in the computer. No check engine light. Could there be a fault in the computer and it needs to be replaced? I have owned the car since it had 22k on it and have never had any trouble with it. I have also changed the plugs, wires, coil packs, fuel pump and fuel filter.