Surges under load

2003 Ford F150, Small triton engine.
90K miles
Replaced plugs, cleaned MAS, New Air Filter, new fuel filter, transmission checked.
At Apprxomiately 1200-1500 RPM there is a hesitation or surge. This is noticed moreso when the truck is under a load. The check engine lite is not coming on, I have had to shops check it, one did a "Live scan", unable to diagnosis the problem Of course this is having an impact on my gas mileage. It was suggested to replace the coils, which are quite costly. My attempt to replace #3 and see if that helps, if not move it to #4 and see if that makes any difference. Very frustrated with not being able to get problem diagnosed or corrected. Any suggestions? Thanks for your time! :Lol: