Fuel mileage is 22.5 mpg during the winter months when using oxygenated fuel here in New Jersey. I am at around 18-19mpg now, spark plugs are white with heavy deposits, not black. The throttle body was cleaned last Feb of 2000, the problem continued, the codes were 231 or 232. I insisted it was the O2 sensor it was changed but to no avail, zero improvement, in total from the Volvo dealer a bill of $157.00 for the throttle body cleaning, and $357.00 for the O2 and installation. Three weeks ago during a cold snap I started the car in the morning, and for the first time it started and stalled twice. The third time I played with the pedal to keep it going, then I drove away... real rough drivability. A high idle surfaced followed by a pulsing upon decelerating by either coasting or braking. I brought it to the dealer, upon diagnosing the found that the O2, mass air and throttle body had passed. The next step was to replace the ECU at a cost of $840.00 with more work to follow if that was not the end to the problem. I bought a Bosch rebuilt unit for $566.00 from another Volvo dealer and installed as directed with disconnecting the battery etc.. No improvement, I went to another dealer they found that the throttle switch was not operating it was changed, $144.00 No change! I cleaned the idle valve some carbon came out, no change, I did my best to look for vacuum leaks. In all the car can perform well if I wipe out the ECU memory and have the car relearn, but after 30-35 min. of driving hiway stop and go, the problems return. And now by coincidence the set button onthe cruise control can be used like an accelerator pedal, hold the set button it in above 25mph and the car the car will take off until you let off the set button the cruise pump just humms under the hood. HELP!!!