Wild guess.
Gas cap?

 

Did you put a new fuel pump relay as well?

 

No new pump relay was added and there was never an issue with my gas cap before

 

Have you checked for exhaust leaks, another thing you might want to check is the per seloniod and charcoal canister if you have that, I have a 96 3.8 with some simular issues and it was the perge flow sensor

 

any update on this???? i am haveing a very similar if not identicle problem.. just started out of blue, doesnt do it when cold, MAF Sensor replaced, fuel filter changed, gas cap checked, fluids checked, nothing seems to be wrong at idel, but when driving under 3kish it does it when pushing on gas, above 3.5 frees up... 1996 Mustang GT 5speed manual only 117xxx miles!

 

Another Guys problem: https://mustangforums.com/forum/4-6l...se-help-2.html

My problem: https://mustangforums.com/forum/4-6l...need-help.html

 

Black boot long 2piece plugs. Acts like water inside grounding out the plugs. Will change plugs and boots to see if problem goes away. Really sounds like spark issue.

 

I had a similar issue changed fuel pump, mass air sensor , injectors and it ended up being few bad vacuum lines. Check every single one follow and Chace them all end to end.

 

Omg I had the same issue and no one new anything about it. I finally figured it out though.

 

I have the exact same issue on my Mustang. Nobody was able to help me out I just kept throwing parts at it. Pretty much going insane pulling my hair out and then I figured out what it was. This is going to be a wing it kind of thing you can try it it might work for you. What I didn't like about this was there is no way to test it. I don't know if your vehicle has one but it was the harmonic balancer for my vehicle. I wasn't able to visually see anything wrong with it I couldn't hear it making a rattling noise after the engine shuts off like normal harmonic balancers break. It just broke enough to become a little bit out of whack throwing everything out of proportion it was horrible and it got worse and worse when my car was cold it ran great when it was hot and warmed up and ran even worse but the higher speed and the higher rpms I got it improved but you could stop tell and feel issues. I was throwing parts added left and right that I'm like fine I'll just changed her mind a bouncer I don't think it fit but it might be home soon as they changed it oh my god it ran perfect I looked at the old Ramada balancer and it looked completely just like the original one that put in I've actually took both came out of the vehicle and paradon they look exactly the same I don't know how it messed up but it did that's something that you can check hopefully it helps you out if it doesn't maybe other people and keep it in mind for their you know vehicles I know the 94 - 2004 Mustangs are pretty much the same course years of the v8 so but I'll let me know if it helps you out. Also I hope this is all spelled and looks correct because I am using voice to text right now my phone.