89 chevy caprice weird engine and drive ability problems?

Description :Chevy 5.0 L / 305 engine [STRANGEST THING EVER] Start the car it idles sloppy for about 10 seconds ,check engine light comes on and stays on while running,it smooths out but has a very light but tell able miss . Put car in reverse and in 5 seconds it stalls. Start it back up wait for the check engine light it smooths back out ,continue in reverse it stalls or drop it in drive it usually stalls again.While running it has half the acceleration a normal running car has, Stop at a light and sit a while it will load up and stall on occasion. Car has very-very poor fuel mileage 10 or less (mpg).It has had a new tune up including new fuel and air filters .

I had it code checked it says it's something to do with the map sensor ,I installed a new one absolutely no change plugged or unplugged .It's something else but damned if I can find it .

I need some expertise with this kind of problem ,I have never encountered this before .Could it be a vacuum leak or a starting to plug catalytic converter .I have checked the fuel pressure and the Throttle Body Injectors all is fine with that part of the system and I have no intake or TBI leaks and timing is correct also.

I need a chevy expert for this car it's my Girlfriends ride friends and I would appreciate some assistance greatly .

I am smart, but I am not CHEVROLET SMART ,Someone has to have the secret GOLDEN ANSWER out there .This car is in pretty good shape for 20yrs old everything works short of the present troubles occuring with it.

All help is very much appreciated ,This is like the perfect problem for a dealership to get in my book.

I have had good ones with FORDS,MERCURY'S,BUICKS,OLDS,DODGE & CHRYSLER,AND A COUPLE OF CHEVY'S .But nothing as wierd as this have I came across. signed :BAFFLED IN BROWNSTOWN,MI.