Hi jakester. Welcome to the forum. I was battling the same intermittent problem with my high-mileage Grand Caravan for almost a year until it finally died completely a few days ago. The problem actually got worse after I replaced the 12 year old spark plugs, wires, cap, and rotor. Turns out it had a shorted coil. Popped a used one in and have driven it over 50 miles with no hint of a problem. My best guess is the problem got worse because I opened up the plug gap a little which makes it require higher voltage from the coil. That coil even caused complete stalling at anything over about half throttle when it was hot outside, similar to a bad accelerator pump on a carburetor. At less than half throttle, the engine would run fine. Funny thing was I could slowly raise engine speed all the way while standing still in neutral. It only cut out when snapping the throttle quickly.
Another thing you might try that only pertains to GMs is to unplug the small connector on the side of the generator and see if the problem clears up. There's no way to sugar-coat it. Starting with 1987 models, GM went from the world's second best generator to the worst pile ever. They are known for developing huge voltage spikes that interfere with various computers' sensor signals. There is a whole list of intermittent driveability problems caused by the generator.
If that has no affect, consider attaching a fuel pressure gauge and sticking it under a wiper arm so you can watch it when the problem occurs. Chrysler pumps tend to not start up when the brushes in the motor are worn, but they almost never quit while they're running. GM pumps are known to quit or slow down while driving. Also, since manifold vacuum goes down when going up a hill, there is less pull on the molecules of fuel through the injector so the pressure regulator raises fuel pressure to keep the forces acting on the fuel constant. If your pump is beginning to get weak, it might not be supplying enough pressure causing a lean condition. As a test, you can unplug the vacuum hose from the regulator, plug the hose, and drive it. You will have higher pressure than normal and probably black smoke form the exhaust from running a little rich, but if the misfire is gone, you might need a new pump. If you see raw fuel in the vacuum hose, the regulator is leaking internally and must be replaced. That is fairly common on GM trucks.
Most people will tell you to check the mass air flow sensor or to clean it but I think you would be having problems at all speeds if he was the culprit.