Possibly other systems connected to the egr valve are faulty. You may need to have a professional diagnose the problem, rather than replacing parts.
no cause a 1994 may have a bigger engine
The normal engine operating temperature for a 1987 Mustang GT is around 200 degrees Fahrenheit. Anything above 220 degrees Fahrenheit might cause the engine to overheat.
Try an ignition switch.
Bad wheel speed sensor or cluster needs replaced
There are pages and pages of things that can cause a check engine light. You need to have it checked with a scantool. Then the code can be diagnosed and repaired.
Pressure test the convertor.
There are pages and pages of things that can cause a check engine light. You need to have it checked with a scantool. Then the code can be diagnosed and repaired.
No You Have Some Other Problem.
No, not if it was done correctly and the engine has not overheated. Overheating is the main cause of a blown head gasket.
You could have ignition problems if it is the engine causing the jerk.
No, there isn't. If the engine runs too hot, you will start to hear squeaks and strange noises from the engine while driving, as the engine metal parts start to expand because of the excessive heat that will cause the engine's interior parts to have a more narrow space to move. This can burn the head gasket, which needs to be replaced or even worst, can eventually kill the engine, which then needs to it's parts replaced and re-tuned to work again.
There are pages and pages of things that can cause a check engine light. You need to have it checked with a scantool. Then the code can be diagnosed and repaired.