Misfires usually show up worse at idle or lower speeds, because the faster you go, the higher the RPM is so eventually it will seem to smooth out. Because the firing is happening so fast that if it still misfires, you can't tell it. Think of a slower guitar part in a song, and then a really fast part. Which one is it going to be more obvious in if the guitarist misses a note?
I've been chasing a similar problem on my '97 Vortec 5.7 crew cab. This truck is lower mileage (so your diagnosis on the valve springs being unequal may apply to a degree here). Since buying it last year it's been driven at least every other day. The truck had old gas when we got it, so I changed the filter pretty quickly and ran Berrymans B12 Chemtool through the first couple of tanks. That helped considerably; the idle was really rough when we first drove it, like "ants in its pants". Next step was new UMP vented distributor cap and rotor, and that made a bunch more difference. Getting it a bit smoother. Couple months later I changed the plugs, put AC iridium 41-993(gapped at .045" I think?). Replacing the plugs with same brand and number as was in it,most likely the original ones. Mo'betta....but not where I want it yet.
Basically I want this truck to idle so smooth you have to look at the tach to be sure it's running... like its brother Burb does(and Burb has twice the miles on its 350!). So I got some additional separators and moved the plug wires around as much as possible, to keep 5 and 7 apart. I probably will change the wires once it's cooler, but they don't appear damaged or feel like they're coming apart internally. Next on the list is a new PCV valve, looking at it today and it's grungier than I like to see them.
Both of these Vortec 350s run great when you get your foot in the throttle, and don't take much throttle to get to 60 or more. And running loaded doesn't make too much difference there.
I've also read some posts on here from others with '97 trucks, that say that some of the "stutter" at idle when warmed up( like a horse snorting and shaking its head) is from the PCM trying to retune the engine to run more efficiently. When it would do this, would be when it's sitting parked, running at Park idle speed which is around 700 RPM,for 10-15 minutes, when waiting on someone or in a drive thru, or in heavy traffic. It does this infrequently now, because I try to keep idling to a minimum because I know how it affects the gas mileage, and I don't think it's good for the truck when it's as hot as it is here this time of year. Was 101° officially today and that's the lowest temperature in 3 weeks.....
I have not replaced the coil or module yet. Distributor shaft felt good and firm, no noticeable play, when I changed the cap and rotor.
