fordtaurus2000

We have a Mustang that has a 3.8l, that blew its head gaskets. Replaced all gaskets,plugs, wires, and heads tested fine. Holds pressure, #1 - 150, #2 - 120 #3 - 120 PSI Has fuel, new injector, new plug, new wires. We ran fuel injector cleaner, and when we ran the cleaner it didn't misfire at all ?!?!? It runs better @ 2k RPM. Coil is fine. I'm really puzzeled. I was thinking that between the fuel injector and the rail, that the little sleve between them could some how be blocked enough under the 2000 RPM, but has enough pressure after the 2000 RPM to start firing. We know fuel is getting to the rail, but we can't get the injector out far enough to see if it is spraying fuel.