I have a 1997 Bonneville SE (Series II L36). I just purchased the car for an awesome price, knowing I would have a little troubleshooting to do. The car has 106,000 miles. I first gave the car some new plugs and wires, as this had never been done. This helped the problem, but did no cure it. So far I have replaced the upper intake, having noticed she was losing some coolant. My father'* Bonne had this same surging problem, and he replaced his upper intake, and has since had no problems. I on the other hand still do. The car surges, generally on a hill-climb. I also noticed that the tachometer will jump erratically as this surging occurs. It does this between 1500 and 2000 rpm'*, with the speedometer reading somewhere between 50 and 65. The tune-up and new upper intake have helped, but they have not cured the problem. Any help would be greatly appreciated. I don't know if I'm having a fuel problem, a bad sensor, or what, but our diagnostics computer won't pick up what'* wrong. Before replacing the plugs, the computer indicated that the number four cylinder was misfiring, but it no longer detects this as a problem since the new plugs. That'* just about everything. Any questions you have or advice you can offer would be great. Thanks.

Try down shifting while going up hill, I know it doesnt sound right but a lot of these cars have a lazy transmission when it comes to moderate hill climbing?
I bet when you put a little more foot into it the surging goes away?

check your coils and plug wires i replaced mine and cured a problem similar to this clean up.