Chevrolet Repair: engine swap, vacuum leak, vacuum lines

Question
I put a 93 350 tbi into an 89 2500 which had a 305. It misses at idle speed and steady throttle but is okay when I hammer on the gas pedal. I have checked timing, MAF, wires, coil, plugs, distibuter cap and rotor, swapped injectors, pressure regulator, ignition module, and checked vacuum lines. I have put a 350 chip from a matching computer (89-90)into the original computer. It occasionally throws a code 42 (ignition module, wires between ECM and ignition module, or ECM). GM told me to change computer chips. Do I need a 93 computer/chip? Can you offer any suggestions?
Thanks  

Answer
Hi Rob, I would be looking for a vacuum leak or a bad/tight valve. The chip doesn't have anything to do with a skip at idle. LOcate the cylinder that is weak or skipping and start looking for the problem there. Spray carb cleaner at the port to check for a leak. Maybe loosen the rockers on that cylinder to see if that helps.

Hope this helps

Mark