2002 Isuzu Rodeo Question Intermittent Rough Shifting

Twice in the last couple weeks my Rodeo has had a relatively harsh/hard jerking when it upshifts or downshifts from 2nd to 3rd or from 3rd to 2nd gear. (At least, I think those are the shifts.) It seems that there's also a less pronounced jerk from 2nd downshifting to 1st.

Both times this has happened, it has lasted for an entire trip. (30-60 minutes.) It did not seem to get any more or less severe as the car warmed up.

In both cases, if I'm recalling correctly, the Rodeo had been sitting for at least a day or two without being driven.

Also, both times, after I've come back to the vehicle, the problem has been gone.

The second time the problem occurred, I was curious about whether just turning the engine off and starting it again would have any effect. So, after driving just a mile or so and noticing the problem had re-occurred, I pulled over and shut everything off. After waiting a minute, I started up, but the problem was still there. However, after driving the rest of the way (to a restaurant), eating and getting back in the car, everything was fine again.

The Rodeo is at the transmission expert's right at this moment, but they cannot find anything wrong. There are no codes from their diagnostics. The fluid doesn't look great, so they are doing a complete flush. (The Owner's Manual says that the V6 model doesn't need transmission service until 120,000 miles, but the transmission tech tells me that the engineers really underestimated this sealed transmission's robustness. And he recommends every 30,000.)

I asked the transmission tech if the range/mode switch or throttle sensor might be involved, since I've seen those things mentioned here (and elsewhere online), but he basically just said that since it's an intermittent issue not giving any diagnostic codes, they have no way to proceed. He said if it happens again after the complete flush, to try to bring it straight to their shop, so that they can hopefully experience it themselves. (I'm not sure what they'd be aware of beyond my observations, but I'll certainly do exactly that.)

Any thoughts? Suggestions? Confirm my tech's approach?

Thanks!