engine cutting out
 

in the mornings when i leave for work and when the temp is between 40 and 55 the engine on my monty seems to not rev up properly. i have to baby it, like give it just a little gas and then let off it and do that over and over to get the rpms up, but if i put it in neutral is revs just fine !! if it push on it it just cuts out and spits and spudders and throws a code p400 (i think). anybody got any ideas ?? thanks in advance..

oh yea 1998 mits montero sport 3.0 120k miles

dont know when the last time spark plugs were changed