1994 C2500, 5.7L V8, 2wd automatic. Truck has low miles (<100k); we bought it 2 years ago. It's had a sporadic issue which seems to be getting more prevalent, but because it's not CONSTANT, I'm not sure how to start diagnosing and fixing it.

About 10x since we've owned the truck, it has not wanted to shift from 2nd to 3rd. Usually this is when under load (we use it to two a horse trailer, about 5,000lbs, well within its capabilities), but not always. I'd say 90% of the time it's while hauling, 10% empty.

Here's how it goes: Start truck up, pull out of driveway. Roads closest to home are wind-y and hilly, so we don't usually go over 25-30mph for 2 miles or so. Truck shifts smoothly from 1st to 2nd, but won't shift out of 2nd gear when under throttle. The instant you take your foot off the gas, you can feel it upshift, but as soon as you apply throttle again, it drops back down to 2nd.

Pull over to the side of the road, turn key to OFF. Turn truck back on, problem immediately resolves, and for the rest of the day, it happily shifts between all gears as expected.

If I take it to my mechanic, I can bet $$$ that it will behave normally; it only seems to act up when I'm a.) hauling, and b.) my trip starts on the very wind-y, hilly roads closest to home-- it seems when the truck has a chance to shift through all gears on the flat, even when hauling, it's just fine. And it never spontaneously starts acting up DURING a trip, only at the very BEGINNING of a trip.

Ideas?