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Please help,
I have a couple of weird problems, i have a 97 gt mustang. The main problem is that out of no where my car will rev up just while cruising down the road on the street i will be going 40 or so and cruising at about 1300 rpm and out of no where it will rev up to 3000 rpm hold for a sec and then drop back down like nothing happend. On the freeway i will be going 70 at 2100 rpm and then it will shoot up to 5000 rpm and then agian just drop back down.
One night on my way home from work my car started sputtering going about 30 then shut all the way off with my check engine and overdrive button flashing which i know is a sign of trasmission troubles but when i turned the car back on the lights were off and the car has driven just like i would any other day. The other is that when idleing at a stop light my idle will drop to low and my car will jump like it wants to take off i know this is the car trying to get my idle up but i hits hard enough that if i didnt have the brake all the way down i would be in the intersection. It has already been through a code scanner and nothing popped up ofcourse. I have already replaced my IAC helped the ideling for a couple of days but seems to be doing it agian

 

Have you tried replacing the TPS? When they go bad, they don't trip a code. I assume you have an automatic and that when the rpms go up, does the car accelerate or just downshift?

 

i havent tried the voltage on the tps thats my next go at it, yes i do have an automatic it does not accelerate just revs up as its shifting down

 

Are you using a Motorcraft PCV valve? That solved most of my idle issues right there. The aftermarket ones just are not that good.

What accessories are running? Defroster? Heater? Fan? Large Sound System, etc? The defroster tends to make my idle jump a bit when im stopped, but never when im driving.

 

Sounds like it could possibly be a torque convertor problem. It wanting to surge forward and bogging down sounds like the convertor is trying to or actually is locking up when its not supposed to. As for revving up while cruising, it could also be the convertor unlocking. Revving from 2K to 5K without acceleration sounds like the tranny could be slipping as well.
I imagine its the same, although I havent actually looked to see on the mustang, there was a switch behind your brake pedel lever that allows the convertor to engage and disengage depending on whether you hit the brakes or not. Normally, as soon as you apply the brakes, the convertor disengages. This is just a guess but I would check to see if your convertor is functioning correctly.