Interesting... I had the opposite experience in my '97s (before any tuning). I tried running 91 octane in both of them and got noticeably worse mileage, plus they felt like they were a little down on power for some reason. Granted this wasn't really an apples to apples comparison as the main reason I tried this was to attempt to run ethanol free 91 from a non top tier station. After switching back to top tier 87 octane the MPG and power came back.
Did you fuel up at a top tier certified station? If so that might have something to do with it. I also went up four points in octane vs your two so that could be part of the difference.
When it was cooler days , the truck would have less of the roughness and surging in the idle. This was a little worse when it started getting warmer. I have the vented distributor cap so I think inside it should be dry....
But Mom and I both noticed after changing grades, that it runs better, rain or shine. Big rainstorm while we were out this afternoon, and it ran great during this. Same as yesterday when it was dry and in the mid 90s.... The gas change has made as much difference as the tune up (cap/rotor/plugs) did. Remember, when we got this one, it had been sitting around a lot, and I had to run a couple tanks and Berrymans B12 Chemtool to get the nasty old gas through it. It's always run strong on acceleration, and quick off the line, but it's never enjoyed idling. Like it's saying, "Mom....you know I wanna get out and stretch my legs, not sit around doing nothing...."