1997 Pontiac Bonneville SE 3800 mileage: 60,000. My car surges between 1700 and 2500 rpms only when the car is completely warmed up and going between speeds of 30-35 then 55-60 or anytime uphill at those low rpms. The car seems to lose power then immediately afterwards will surge back to the rpm it should be. The dealer has replaced the fuel filter, plugs, coils, wires, and air filter, serviced the fuel injectors and replaced the torque converter. They said they now want to replace the transmission to see if that fixes the problem. My guess is the engine is next. No error codes are displayed in their diagnostics. So, they are "experimenting" until they find it or I run out of money. I have already spent over $2,000.00 for these "services". I noticed someone else with a similar question to you with same car and you suggested EGR valve. Any ideas?
We would first check the fuel pump to make sure the pressure is within range. If this is okay, try replacing the oxygen and coolant sensors. Sometimes, computers do not set error codes unless components totally fail. In other words, these sensors could be out of range for proper operation of your car without setting a code.