Hard to start when warm is a good indication of Bad OVP relay. Does your ABS light comes on every so often?, If it does, I put my money on OVP.
Another may be your fuel pump relay.
The "governor" the mechanic/dealer mentions must be the EHA valve, I doubt that is the problem.
"Many components have been replaced", that does not help much.
Try to be more specific: changed distributor cap and rotor, changed OVP, etc..
Do a search on both. Tons of information.
Good Luck, and keep us posted.
1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car