Same problem new motor..

[TrueChevyTech]

87 tbi 350
Engine idles fine hot or cold, but when warmed up if i ever so slightly push the gas the engine will stumble, although i can accelerate right out of it without hesitation. this was the main reason I spent a load of cash and had it rebuilt.. either way the motor was wore pretty bad so no wasted dollars there.
I get error 33 out of my check engine light which refers to map sensor i've been researching test methods although it seems I don't have a friend with a vacuum pump, any other ideas?
also the oil pressure gauge goes way down to 15 psi at idle.. i'm going to hook up a manual gauge to test it and just use the stock one as a reference .. that sounds like the correct first approach to me.