when the engine heats, a crack in the Crankshaft sensor gets wider until it looses charge, and it goes fast.. within a couple seconds. Thats a possibility. Have him check the crankshaft position sensor and all accociated circiutry...
A 1990, if fuel injected... check the injector(s) as they could act up when warm as well, and effect the current of the fuel injection circuit in such a way that they cause the circuit to shut down... usually once cool the car can start again. Tell him to check the resistance of the injector(s) when cold and when hot and compare to specs.
Intermitten codes are "soft codes" which are retrievable because the computer will hold on to them in memory for so many drive cycles... go to a different mechanic.
Im not familiar with the custom cruiser.. i dont even know if it has throttle body injetion, or if it is a carberated engine.. throttle body injection looks like a carb though... are you certain of what you have??
How many kilometers or miles do you have on the engine, and when was your last full service?