This one has me pulling my hair out..........

Start up and idling, it shows MAP of 38.93 kPa, going down the road it gives me similiar readings during a steady state light throttle cruise.
But I noticed the other day that when I start to feed it more throttle, the MAP values will go up to 50-60ish, then start to FALL.......then in turn affects my fuel and spark maps, and as a result the truck runs like crap.
I replaced the MAP sensor not too long ago, so I'm trying to chase down what could cause the problem.
I looked under the truck and the catalytic converter is still there......might it be stopped up and causing a back pressure issue that is showing up as wacked out MAP readings?

 

I'd hook a vacuum gauge to the line going to the map, see how smooth the needle is moving. Should have 18" or more at idle w/ a smooth needle.