Intermittent misfire 89chevy5.7TBI after pulling 6500 lbs trailer for 70 miles

I have a 1989 c1500 5.7 TBI truck that I use for pulling a race trailer. Only after pulling the trailer @ speeds above 65 mph for a distance of 70 miles or more does this misfire occur. I have owned this truck for less then a year and been hunting the cause of this misfire pretty much the whole time. Before pulling anything for the first time I took the distributor out and installed a new pickup coil, module, cap and rotor. Lubed, installed and set timing with a light. After returning home from the first trip and experiencing this misfire my hunting began. Dragging out the picoscope and and snap-on scanner and OBD1 adapter. No codes, fuel trims all good, very good rich O2 reading @ WOT going down the road. So we move on to hooking up Mr. Pico. After looking @ everything that I can think of - I find a random High secondary KV at idle and a strange injector voltage waveform ( never in my years have I seen anything like this ) - I found the time to get the wiring grommet free out of the throttle body to gain enough wire length to swap connectors on the injectors and the strange waveform stayed with or followed the wiring , not the injector. both injectors OHM out at 2.1 Take a look at the waveform and maybe someone will remember seeing something like this before. Mind you this engine runs sweet at all other times and I have replaced a LOT of other parts for other reasons that I have not told you about. It has 189K and when I pulled the valve covers to replace stem seals and springs - you could eat off the inside of it. Only other thing I can tell you, is that after sitting engine off for 2 - 3 hours after the misfiring, it goes back to running sweet again without fail. So I have never been able to do any testing while it has the misfire. Thanks ahead of time for your comments, I'm guessing bad driver in the ECU.

Did you ohm the primary and secondary side of coil?

If not ohm it cold and hot same with injectors.

Does it continue to misfire when you reach your destination, or is it only perceptible while pulling the trailer?
My first thought was torque converter shudder while pulling the trailer.

Actually to expand on noahs question are you towing in overdrive? Versus tow haul mode/not overdrive?

First off Thank you for your time. I have not ohmed the injectors cold, but will. The ignition coil was first changed to a new standard motor products - with the same results. And now has a new A/C Delco on it and does have the capability of producing a little over 32KV under testing conditions. Were you able to look at the waveform of the one injector ?

Still learning my way around this forum. This is the 3rd truck that I have used for this purpose all with 5.7 TBI engines and 2 of them with THM 700 4R and one with a 4l60E. I always pull in 4th gear or overdrive with lock-up, this engines are great low end torque producers and get awesome fuel milage. Not bragging but these trucks when running correctly will pull my rig easily 100mph down the highway.

All but 2 times has it continued to misfire until I reach my destination, but on the 2 occasions I was running in drive three tapping the brake pedal intermittently to keep it out of lockup trying to keep it at at least 45mph ( nursing it ) and all of a sudden it smoothed out and I at first ( cautiously ) feed it throttle and shift into 4th gear and all is fine. Thank you for your time.

My apologies for the late response, i have been a tad busy.

Ok the reason i ask is because you will burn the tranny up doing that in overdrive.

As for the waveform i will have to see about a downloading picoscope software to read it.

I would say with the random misfires you might want to scope the ignition control module in the distributor as well as put scope on the iat and ect sensors on your next long haul. Otherwise corrosion at terminals would be the other place to look.

Was it raining when it acted up

Thank you for your time and reply sir, when I first got this truck - before I ever pulled out of town, I replaced everything in the ignition system from pickup coil to plugs. The module that I used at that point was a standard motor products. But as this problem kept coming up while towing and bugging the crap out of me - I started scoping everything and did not like how the ignition ( primary and secondary ) waveforms looked at the point where the module grounds the coil, it was just different. and decided to purchase and install a genuine GM module and what a difference - picture perfect on the scope. And I did go back and add a screen shot of the waveform so people could see it without the pico download. I have since found out the this strange peek and hold injector WF is caused by a bad capacitor in the ECM and is called Ringing.

Very cool glad its fixed