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Hesitation in mornings

For the past 2 mornings, my 97 GMC Sierra pick-up has had a slight hesitation when accelerating. After it runs a while, there’s no problem. In the afternoons, it doesn’t do this. It is at the shop now & of course, if didn’t do it while they had it, so they are still checking for problems. Any ideas? Thanks

I assume no check engine light?
Has the manual recommended maintenance (filters, plugs etc.) been done? That’s where to start.

From there put a bottle of Chevron or Sea Foam in the gas tank.
Get the ignition system checked out.
Check the fuel pressure.
Get a compression test.
Get the head gaskets tested.

Hopefully it will be fixed before you reach the bottom of the list.

There’s something wrong in the cold start system, like the coolant temperature sensor or the Idle Air Control. You may need to visit a dealer or other mechanic who is familiar with these trucks.

Thanks, they replaced the oxygen sensor today. We’ll see in the morning if that works. It was fine on the way home.

Has it been damp in the mornings? How old are those spark plug wires?