Engine New problem, hesitating on acceleration and hard starting

Did the Plug Wires fix the backfire and hard start issues?

Did you do a compression test? Make sure all of the plugs are out when you do. That way if you have a head gasket failure between 2 cylinders it will be easier to detect.

Common Things to check...
Head Gasket.
Fuel Filter
Fuel Pump. ( This is the sneaky one that gets past a lot of mechanics. )
Valve Train.
Temp Sensor for ECU.

Computer control systems rarely cause a backfire with the exception of The Temp Sensor for the ECU. Or the ECU Being Bad. Rough Idle and backfire are usually cause by Valve train issues (Worn Valves, Sticking Valves, Worn valve Springs, Worn Timing Chain), Head gasket blown between 2 cylinders, Or incorrect fuel air mixture.

I have seen 500K miles on a couple of stock daily drivers but they were not a Mustang. One was an Acura and the other was a Chevy Pickup.

Original Clutch with that many miles is impressive. Sounds like you take really good care of it.

Lower end is probably getting a little sloppy with that many miles. But if it was to the point of being an issue you would know it. It would be making a lot of noise when it was cold and kinda clear up once it warmed up. Or you would be knocking and ticking a lot.
I did get the coil and plug wires changed. It seemed to make it easier to start, but the hesitation is still there.
I have not checked the compression yet. I will either rent or buy a compression kit. with the number of cars I have, I will probably need it often.
I will replace the throttle position sensor, and maybe the ACT. The TPS is not original, but the ACT is.
On cold starts, and when I change oil, I don't hear much from the lower end, so I think it is OK for now.
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I had a Merkur that had surging and idle issues right before I sold it.. I chased it for a few months and could not figure it out.. The person who I sold it to finally fixed it, said it was a PLUGGED vacuum line.... Might be an area to look at...
I know it's already been mentioned but it's worth checking the EEC capacitors if you are having hard starts. There's 3 of them. There are tons of YouTube vids showing this....I have a folder full of them...
With the age on our cars it's a good idea to check them anyway. The leakage will destroy the board if it's too bad. Better to replace them and get another 30 years out of it.
I had this happen to my Airbag Controller. The caps puked out their electrolyte and it ate away the copper traces. Being an electrical engineer, this was an easy fix. I used much higher quality caps, rebuilt the missing traces, and conformal coated the board. This failure will not happen again on this board. If I get a chance, I may pull the ECO and check it out. I do know that aluminum electrolytic caps have a specific service life. They don't last for ever.
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Did some more work this weekend. Changed and adjusted the TPS. Might be slightly better, but still hesitates and backfires on acceleration below 2000.

Compression results look good:
1: 165
2: 170
3: 160
4: 170
5: 170
6: 165
7: 165
8: 175

Also changed oil, but I don't expect that to matter.

I did "try" to change the IAC sensor, as the current one is original, but RockAuto sent the wrong one. Looks close, but would not mate with the connector.

Another thought, the TFI is original, could it be jacking with timing? I would think it would be consistent across all RPM and load conditions. I have a new one. Might try it, if I can find it.
Nowhere here have I read any real diagnostic work done except a compression test. Go to the technical/how to thread that I linked a page ago and look for the 'surging idle checklist ' do the steps one by one, don't skip around.
Or you can hope this part or that part will fix it.
Trying to fix it the way you're going could cause more issues. Aftermarket parts are hit and miss. Very easy to end up chasing your tail by " shot gunning " parts.

You know the compression is good. That's a really good thing. The pick up assembly inside the distributor can cause a stumble, the TFI can cause a stumble, injectors can cause a stumble, a vacuum leak can cause a stumble, the salt and pepper shakers can cause a stumble, a loose wiring connection somewhere else can cause a stumble, a bad ground can cause a stumble, a bad IAT, ECT, ECU, TPS and...many other things can cause a stumble. Just because a part is new doesn't mean it works properly.

Most parts are made in one of two places. Either Mexico or China. We've made both of them mad over the last few years and part quality has gone way down.

Diagnose the problem with some simple cheap tools and you'll find your problem before you spend hundreds of bucks.
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