The intake manifold gasket is probably cracked (super common, I've had mine replace 3x).
When you put in gear cold the engine has a little more load on it and it is sucking air from the cracks in the gasket.
Once it warms up the cracks seal up a bit and it runs fine.
I think an old isuzu dealer (or Honda) dealer would charge about 350 to change one the gaskets.
Isuzu have bad tranny's if the owner don't change the fluid. The like 25-35k fluid swaps. I have 120k on my tranny and have never had an issue.