1986 MustangLX 5.0 engine problems

Hi all, new to the site but long time browser. Been building and working on Foxbody Mustang's for over 15yrs in my spare time as a hobby. I broke a torque box last fall and the car sat all winter with very little gas in it. Once I fixed the box and took the car for a drive I noticed something very odd. When the car is ice cold everything is fine. After about 3 minutes it goes real bad. It bucks and spits and pops all the way through the rpm band and when you let off the throttle huge backfires occur out the tailpipes. I thought this was just an under load problem but if you rev it in neutral around 4500rpm it starts to break up all the way to 5500. Trying to go WOT in any gear just doesn't feel or sound very good with the sputtering,backfiring,bogging,etc. I have spent a lot of time researching this problem and have troubleshot it to death. Have had the motor down to the heads twice so far checking everything mechanical,electrical and in between. It's not wires,plugs,Distributor,cap,rotor,TFI coil,vacuum leak,timing,fuelpressure,lifters,rockers,pushrods, hoses, frayed wires. I did KOEO and KOER tests with a test light and paperclip. I have removed the smog pump and AC so most of the codes had to do with that.

KOEO CODES 81,82,85
KOER Codes- 94,44,33,99

Again, for the first 2 minutes car runs like a dream. I try not to get on it as we all know mustang's like to be warmed up first. After that it bucks,spits,sputters,backfires,and bogs. At idle you can hear the engine stutter if you stand near the exhaust. It does this every few seconds then returns to normal idle.My mods are as follows...

1986 MustangLX 302 5spd. Speed Density car, NO MAF
Trick Flow street/strip intake-BBK 255lph Fuel Pump-BBK shorty headers-2 1/2" free flow cats-H-pipe to SLP resonators-1.7 crane cams roller rockers-ford racing pushrods-CAI with enormous cone filter-removed smog&A/C running short belt-3.73 gears-ADS Superchip-short throw shifter-Hotchkis suspension-3 core radiator. stock heads,stock cam,stock block.

I've had these mods for years and the car has always ran like a champ.
My personal conclusion is the fuel return line may be clogged up forcing the fuel pump to overheat after a couple of minutes. The problem with that theory is my fuel pressure gauge holds a steady 42lbs at WOT under load even when its breaking up badly. Would love to hear anyone else's theories. Nearly stumped and running out of options.

Here is the car running properly. I will put a video up tomorrow of it running badly.
//www.youtube.com/watch?v=jTO4Ef6iw2E

There were no changes made prior to the problem except reinforcing the frame and welding the torque box back in. Things I have tried... Lifter test,changed pushrods,plugs,wires,cap,rotor,checked all vacuum hoses,checked intake for any vacuum leaks,checked all electrical connections,checked timing/fuel pressure, replaced TFI coil. Pulled KOEO and KOER codes. Even tried switching out EEC for another I had.

Update: the last KOER test was the cylinder balance test which ran twice getting a 9 each time which clears that of any issue.

167,000 members and 1 reply in 4 days? Guess I will try a better mustang forum but thanks for your time or lack thereof lol

AS long as you don't do cam or large heads it is ok to have basic bolt ons and run fine for many years. Being as I have had all these mods for over 4 years I can attest to this being true. Sure I can't make big power but, I drive the car nearly everyday and she is pretty quick. Well, up until she sat all winter that is.

UPDATE: I have replaced everything in the ignition system except the distributor which is on it's way. Also replaced O2 sensors. The car still runs great for the first 2 minutes then progressively worse as it warms up to the point of nearly undriveable.