Isuzu trooper starts fine runs good once you turn it off hard to restart almost like it's flooded some times will backfire . And has had the reduced power come on two times. I had it at three shop

id give back that ring..... LOL !!!!!

we can not answer, unless
1: country stated, usa or canada is my guess.
2: no engines stated, of the 4 opted day one sales.
3: no year car, wow , now that makes this 10000 harder.
sounds (Im, deaf) like the cam was timed wrong to me.
or if old car, the distributor and cam timed wrong

are you building engines book free.
why do that. pure ttorture?

backfiring means, "You timed me wrong"
alldata.com covers this, (usa only)
here it is, about price of gasket.

dirt cheap FSM like books online, #1 quality

https://www.scantool.net/software/alldata-diy-1-month-subscription.html

no commissions from alldata. it's just good, source.
im retired, and help the helpless.

Have a test done on the glo-plug system as it appears that they have burnt out and are not pre-heating the cylinders or providing a source of ignition for the atomised fuel from the injectors

Great.

Location is lower right side of engine block. ,you also have to remeber you have the famous ICM (ignition control module) This connector is famous for loose or corroded terminal, its located behind ICM at the intake manifold.wiggle it see if that will help to get it started,wiggle the connector you crank have your brother wiggle the wire to ICM and even the cranksensor wire

Replace the coolant temperature sensor. This part determines the operating temperature of the vehicle, which the computer uses to determine the correct amount of fuel to the injectors. When the sensor goes bad the computer thinks the engine is cold all the time, so it sends too much fuel into the firing chambers, basically causing it to flood.This will cause the vehicle to run fine when the engine is cold, but once warmed to operating temperature it will stutter and lurch while driving, and even die at stoplights sometimes. I'm 99% sure this is your problem, although it could be a bad oxygen sensor.
Although these parts aren't necessarily maintenance items, they do fail at some point, as the engine gets more miles on it. The parts are relatively inexpensive, in the $20-$30 range, so you're not out anything by just replacing them.

Have you cleaned the throttle valve and the air bypass idle speed control valve? If not do that. When they get really gummed up from backwash from the combustion process they cause this problem of stall on cold start.

I have the same truck, my motor went south blew the head gaskets, but right befor that i was having the same issue. It qwas a bad spark pulg and a O2 Sensor, i just changed the plugs and it was all good after that.

good luck

check fuel pressure possibly too low fuel pump not pumping enough

If its the 3.0 diesel it could be the rail oil pressure sensor starting to pack up .

have someone kick at the gas tank while you are turning key if it starts it is fuel pump