[DanEC]

If he has an electronic ignition the engine is probably cutting out because the idle rpm and accessories are pulling the voltage below the minimum the ignition box requires to function. Is he running a battery gage (some call an amp gage)? If it's going way negative when slowing to a stop or corner with all the accessories on then that would indicate the voltage output of the alternator/regulator is dropping way below what it needs to charge the battery - which means it's probably too low to operate his ignition box.