'01 Montana slow upshifts after head gasket job

[JPHarr]

I just got my 2001 Montana back from a "reputable" shop; had intake gaskets replaced, as well as intake gaskets, spark plugs and wires, etc.
A new problem started since getting the van back - slow upshifts. Takes a long time to engage 1-2, 2-3, and Overdrive. They had not disconnected the battery, so that's not it. But they were completely sloppy in making final wire and other connections. Loose band clamps, mis-routed wiring, mis-routed hood release cable, and the kicker: a loose big red wire atop the alternator. That's a whole different fiasco late last night.
Anyway, because of the sudden appearance of the slow upshifts, which I am sure is because of something the shop did, I strongly suspect a missing or loose electrical or vacuum connection somewhere. My research so far this morning indicates a likely vacuum leak someplace.
There were no transmission troubles before I took it in for the head gaskets. Any help, anybody?

[JPHarr]

A lot of online research later, I was only finding info pertaining to a failure of unknown origin. My problem is 99% likely caused by the shop who did the head gasket repairs.
So after posting here, I got the advice to reset the computer again. I did so, pulling the positive battery cable off, yanking battery and ECM fuses, then pressing the horn and brake pedal to ensure the system was drained. I also double-checked every vacuum and electrical connection I could see and reach.
After about twenty minutes I put the battery cable back on. Strangely, my radio settings had been retained. I found that after the shop repairs, too, but maybe they had pulled battery power after all (which does make sense).
Finally, I took the van on a couple of twenty mile drives, both around the city and on the highway. Result: the transmission is shifting normally. I cannot explain it, and so am not completely convinced that the problem is resolved.

[balmer_guy]

Interesting! I'm going to try that on my 05

[JPHarr]

That was it. The van has been shifting completely normally after pulling the positive battery cable off, yanking the two big battery fuses and the ECM fuse from the underhood fuse block, and stepping on the brake pedal a few times (this to dissipate any lingering voltage).