1991 Mighty max 2.4 miss firing/hiccups and 45mph to 60mph

Hi im new to the forums and was looking to see hopefully someone that knows way more about the 2.4 4g64 could point me in the right direction to start diagnosing this goofy problem just bought this 1991 mighty max all original airzona truck not a spec of rust or rot anywhere just got the transmission rebuilt as well but the issue im having right now is that if i cruise normal city speeds truck is phenomenal but running at 45 to 60mph range only at part throttle itll hiccup and sputter i just did plugs wires and cap and rotor since all of it was original to the truck im not too sure where to exactly start diagnosing any help and guidance is greatly appreciated

Welcome here, other members can help but I'm only good for wanting to know how you plan to keep your rust free truck rust free on salted roads?

Clean clean clean theres plenty of ways to keep em clean up north most of this winter trucks been stored anyways

Next would be the ignition coil. They break down after 10 years of regular driving and it sounds like the symptom you'd get if the coil was tired.

if you have an oem coil on it its probably time to replace it now anyways. check the ecm if the caps are leaking. post a picture of its innards.

What are the caps on the ecm if you dont mind me asking

Capacitors on the circuit board of the electronic injection control module. The ones to examine look like tiny cylindrical cans. Sometimes bad ones are obviously swollen or ruptured, other times they just leak out the bottom and can only be spotted by the crusty residue they leave on the board.

Thank you i will for sure take photos and ect for you guys i wanna say about 95% of this truck is original so anything common that i should go over with this truck that i need to know would be great i did plugs wires and cap and rotor did have a new coil from rock auto but the stud broke off while i was tightening the wiring so ill have to start at least with a coil and see if that changes performance at higher speeds

Bad grounds are a common source of electrical issues so go through and thoroughly clean bolts and connections to the frame, body and engine. Same with corrosion in the main battery leads. Coil packs have a life span of about 10 years of daily driving before they show signs of breaking down. Timing belt failures from age and fatigue are common and should be replaced every 75,000 miles maximum or 10 years if infrequently driven. ECU failures are common due to the type of capacitors used in them and should be checked before they do any damage to the circuit board (this goes especially for the 1990 year model trucks as they have a year specific ECU and ancillary engine sensors/controls) Cooling system maintenance is more critical to the 4G64 2.4 engines as some cylinder heads are known to be less resilient to overheating events and have been known to cause cams binding in their journals to the point of actually snapping them in half. Most oil consumption and exhaust smoke is contributed to bad valve stem seals which have the same life span as coils and timing belts (approx 10 years unless the engine has been serviced more frequently) It doesn't hurt to clean out the IACV on the intake manifold to maintain good function and if the engine has never been through an oil flush, this will go a long way to prolonging oil quality and keeping the hydraulics fully actuating (an oil flush can reduce or even eliminate noisy valve lifters and restore power by returning valve lift to it's full travel)