Engine randomly shuts off

[87chevyk10]

Hi, i am working on a 89 305 tpi for a guy. he said that when he is driving it randomly shuts off and is very inconsistent when it does shut off. he can not guess when its about to shut off. after it shuts off it wont start up until 5 minutes later. here is a list of things he changed or did:
tune up
fuel filter
distributor
Fuel injectors
Fuel pressure regulator
strainer/ float sending
Ecm (twice)
ignition switch
02 sensor
tps
egr valve
maf sensor
lac valve
ignition module
coil
2 relays
so can you guys help me out here and tell me what possibly it could be?
After it shuts off it starts up again but the idle goes up and down. when u hit the gas it shuts off. and when you start it up again the idle still will go up and down but when u hit the gas it would stay running. And the guy i am working on this for replaced all the things listed above by a garage and by his self. And now i am trying to figure this out for him. he took it to a couple of garages and they dont know.