2004 Oldsmobile Alero hesitation/poor performance

Engine Performance problem
2004 Oldsmobile Alero 6 cyl Front Wheel Drive Automatic

hi guys I hope you can help me. My alero hesitates
and kind of surges while traveling on the highway. She starts up fine and runs good but after traveling a few miles on the hwy the car rpm drops (about 3-400 rpm) and back up again. This continues until I pull over off the road. While in park I raise the rpm and notice the surging continues at 1200-2100 rpm. As I start to move again it runs like that even at slower speed. I tookthe car to the dealership (by the way no check engine light/service engine soon light comes on) and they checked for codes (none found) and ran a diagnostics with no problems. The y changed fuel filter and "flushed" the injectors. Later that day it sarted happening again. I changed all plugs and wires and no change. I just replaced fuel pressure regulator and still no change. I dont want to take it back to dealership because no codes are showing and their way of troubleshooting is to replace items instead of trying to properly t/s and repair. This will become very expensive and I cant afford that. Please help! Thanks!