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98 mustang gt with major engine hesitation/loss of power

I have a 98 mustang gt 4.6l v8 all stock parts, besides the exhaust, which is a straight x pipe. My engine seems to be loosing alot of power, if i accelerate past 3500rpm my engine will start to hesitate and stumble, and if i go to wide open throttle while the engine is stumbling i will loose all power and my engine will drop to about 1500rpm and will begin jumping…almost as if i am pulsing the throttle. but the wierd part is if it starts to hesitate and i turn my engine one and off it will regain power, until i go wide open throttle and go past 3500rpm. my first thought was my fuel delivery, so i installed a inline fuel pump right after my inline fuel filer by the gas tank. i put the second fuel pump on a toggle switch so i can turn it off and on. when my car begins to hesistate i flip the switch to turn on the second fuel pump and its like a shot of NOS and i get 100% power back for a few miles then it begins to hesitate and stumble HEAVILY. I do get one engine code P1443 “EVAC Purge valve fault”. The engine will only hesitate under load, if i put in my clutch and rev it will seem fine, until i put load on engine, it will begin to stumble. it seems to be getting progressively worse everyday…please help. and cheap ideas would be good because I’m a broke college student hah, but i need to get my pony fixed. thank you for your time, feel free to email me at

Throttle body/MAF sensor?

I have an '01 GT model that was hesitating / jumping rpms when the weather got colder. If I stomped the gas when it hesitated (thinking it was lack of fuel) it got worse and sometimes stalled. A friend suggested a new mass air meter. I replaced mine with one from Pro-M and haven’t had any further trouble. I’d give it a shot.

Could be the MAF. YOu can clean the wire with spray electronics cleaner. I would consider the TPS as well.

BTW, since when is NOS “stock” on GTs?

I sometimes have the same issue. I have pinpointed it to being ETHANOL. The cheaper the gas and the more ethanol it has in it the worse my 99 mustang runs.