When's the last time he changed the fuel filter? I was beating my head trying to figure out a hesitation a week ago and it turned out to be as simple as the fuel filter. It had about 50k on it.
A rough idle could also mean a dirty IAB valve. Clean it out. It can also affect operation at cruise.
A stuck open EGR valve could cause such problems. You may want to try cleaning that out.
A fouled ACT sensor could cause this sort of problem.
If it's running rich, it may be bad O2 sensors. Have they ever been replaced?
Have you pulled your codes? The problem may be stored there.
Just some ideas.
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351W 89 Mustang GT Convertible