Other way round, early AE map had cold start issues, later revised AF map had a 4K "cough". It's only a brief missfire, some cars do it some don't.

First things to check are plugs & leads, check the plug gap and gap as necccessary. I run at 0.7 gap, any more than that gives a missfire on boost. There's lots of debate on what plugs should be used, some tuners recommend using std Ford plugs, some prefer Denso Iridiums. I tried the Iridiums in mine and had no probs but there were a few people who had problems with them dropping the centre electrode, probably caused by ham-fisted gapping technique more than a fault with the plug, I switched back to std Ford plugs and have had no problems running 300+.

Also, as you don't know the history, I would check how much boost you're running, connect up a temporary boost guage if necessary.