my car idles fine until after 30 mins of driving. after that in drive the idle shows 900 instead of 6-700 and in park jumps up to 1300 and settles to 1000. while i am driving the gas pedal feels different every time i press it. what would cause this????? i have replaced egr, iac, tp, ect, spark plugs, rotor, dist cap, wires, intake manifold gasket, tranny filter service, throttle body is clean and have reset base idle vacuum is at 17 at idle and fuel pressure is 35. this is an 88 mustang gt aod and speed density. any help would be appreciated running out of ideas for things to fix