99 Tahoe Rough idle in closed Loop

Help please!
Hey guys ,hoping to get some help on a problem im having with my 99 tahoe. I bought it cheap for the winter but It has a P0300 and P0171 code. Which ive been messing with for a couple of days now. I cleaned out the MAF,replaced the IAT because it was cheap enough,cleaned out the Map sensor, and have already done a tuneup on it. Truck starts right up, i mean Right up with no hesitstion and runs perfect until it warms up. Once it hits closed loop,the idle drops real low,misfires like crazy,and the fuel trims go thru the roof sometimes 50% or more if you count the STFT. However, sometimes it kicks itself out of closed loop and back into open and runs fine again. I checked for vac leaks just for fun and didnt find any. I should mentionx after i cleaned the MAF ..it Seeemed to run fine for about 45 min . I shut it off. Turned it back on the next morning and it started doing it again. Also should mention the CAT is disconnected at the Apipe because the previous owner thought the cat was clogged and tried cleaning it out. But this problem was present then as well or so he said. Any ideas?