dave the 2 plugs your talking about are the crank sensor and knock sensor plugs.and she defo wont start with crank sensor in the wrong one .your clearances are correct,
the symptoms you discribe are more than likely a sensor failure,the p8 ecu on the big turbo escort cozzy has a built in protection that causes the engine to limit and splutter at about 3500rpm for a bad signal or sensor failure. its to protect the engine,sometimes you can drive through the 3500 barrier,you may even get it to happen just by reving the engine to this rpm without moving.
have seen this before,happend to a mate after a turbo change,we scratched our heads and thanks to stu,s wisdom we realised it was the lambda probe,once replaced,problem sorted.
so start checking them all,and you may find the cause