When i purchased this 97 diamante, i gave it a tune up. i know my mecanic, he is not a con artist. in fact we work together on the car.so its not him, just for you knoledege. After the tune up it had great gas mileage, for maybe 3 days.the car would accelerate all the way to the 2(bad car language,x 1000 rpm maybe)on its own, whether its in park or neutral.it wasnt constant but it happened off and on very often. As i drive, its like the car is fighting me when i brake. and it would overheat. over time , i noticed that when the idle(i think)went out of wack, then the temprature(overheating) would go out of wack.i would turn the car off when the acceleration and temperature go crazy. somtimes it would start back up and sometime it needed about 5-10 mins before it could start. AND THE CAR WAS BURNING SO MUCH GAS. the check engine code it kept giving was p0400. i didnt want the dealer to kill my pockets so i asked them question on what the propblem might be. first iwas told the egr valve behind the engine. changed it and nothing happened. then i was told maybe the soleniods. nothing happened. I finally changed the idle speed contol motor. what it did was lessen the times the car went crazy. it did not solve the problem. it still accelerates on its own, just not as often and severly. and the temperature hasnt gone all the way to red, just 75% if u know what i mean.and agai it was not constant.off and on. I would greatly apprciate if you could stop my wild goose chase? this car runs very well despite this issue. My main issue is the gas consumption.7-8 gallons Would be finish in just 62 miles. and that hurts when its super or premium. in the tune up, i changed everthing. spark plugs, fuel and air filter, pcv,D.cap and wires. after the tune up, there was a slight issue with the wrong order of the 3 color coded wires(clear,red and green stripe) on top of the engine that caused the car to shake, but that was taken care of. I appologize for this long letter, but i wanted to make sure i left nothing out so u can give a straight solution. Thank you in advance.