Common fault involving the shift delay when engine i cold. It is there to help warm up the Cat fast from a cold start. Behind the booster is the SOV valve that operates the element down at the trans that holds the shift in the lower gear.
You can pull a vac on that green vac line going to the trans w/hand vac. pump and see if it holds vac. Also see if that or the other vac lines are broken/cracked/leaking...
There is another test, but try that first.

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A Dalton