If your MAP sensor was bad, it would show a CEL, so that isn't your problem. The engine will have a hard time without the MAP sensor working because it relies on that as a signal to pull fuel or add fuel and ignition timing.

Another thing you may want to check as it just popped into my head is the ignition timing. Hook up a timing light and make sure that the distributor timing is to stock specs. I know that I was having a stumble under boost and the ignition timing was retarded beyond stock a few degrees, so it may be another place to look.