1996 cranks but wont fire

1996 SLT 4.3 vortec 160k, No codes
Problem started out of the blue after work one realy cold day (-18c) where the truck cranked over but would not start. Eventually had to boost it and even dumped abit of fuel into the body. Before that it started perfect every time. Since then (each morning, extreamly cold) cranks and cranks and cranks and eventually it will start. Once it starts and runs for even a few seconds you could turn it off and it would start right back up no problems, even after sitting for a few hours (till lunch) it would fire up instantly.

Changed plugs, air-filter, oil, fuel filter, pcv valve, main coil ignition wire, ignition coil. All had exactly no effect, plugging in the block heater seemed like it might have helped abit until I realized that during the spark-plug change (behind the steering column) I must have knocked the plug from the block so it wasn't actually working at all. Tried to disconnect the IAT sensor before starting in the moring and it seemed to work better so put a new IAT in, seemed to help for a few days but didn't last.

Now it starts hard in the morning and randomly every other time, drive for a few minutes or 2 hours, turn it off to run into a store for a second, come out and either it will start instantly or crank and crank and crank. warm - cold doesn't matter any more.

When the key is turned on you can hear the click of the relay from over near the glove box then a few seconds later another click (fuel pump priming?) around back you can hear the pump running. After cranking for a few seconds and then turning the key off you can hear the pump running for a few seconds longer.

The only way to get it to start now (when it wont fire on the first try) is to keep turning the key off then crank then off then crank...it might give a small chug where it tries to start once but if it doens't start you can crank until the battery dies it wont do it again until you cycle the key off and on again.

When it does start it doens't seem to puff and smoke or idle up or anything so it doesn't seem like it's flooding (making it seem like a ignition problem, some sensor not allowing it to fire) or some other sensor not firing the injectors?
Out of pure frustration (brute force and ignorance), tried pouring a little gas into the whatever-that-thing-that-replaced-the-carb-is and it fires right away so i'm thinking the ignition is ok so I'm back to fuel.

Touching the gas pedal has no effect (abit, half or full)
When you turn the key on you can hear a electric-gear motor up near the throttle body adjusting something (Choke type bypass?)
Other than a very slight hesitation comming off idle it runs fine, no gas smells, milage seems normal, maybe abit slugish but it always seemed to be. Slight bend in the steering wheel is from my head banging on it.

Fuel Pump? Pressure Regulator? IAC sensor?
Any takers?

check your fuel pressure. sounds like pressure is bleeding off when the engine is off. this normaly happens when the pump checkvalves are bad or the regulator is bad. most times it's the pump. if you decide to go after the pump make sure the tank is on E this makes this a lot easier. whenever you replace a pump put a new sock filter on the pump and replace the inline filter. good luck

Pump shows 40 psi with the key on, will go to 60 by cycling the key on - off a few time but drops back to about 40-45 when you try to turn it over.
Wouldn't start tonight at the store where I bought the pressure gage on the way home from work, got to try it there in the parking lot. In the morning have to let the garage have a try

i'm not 100% sure of the fuel pressure required for the 4.3 but i know on the trucks if there isn't at least 58-59 the injectors won't work. maybe call a gm shop and ask them. try pinching the fuel return line at the rubber part with some vise grips and see what the pressure does. i don't recomend driving with vise grips on it but if the pressure stays around 60 you know the regulators bad.

Pump shows 40 psi with the key on, will go to 60 by cycling the key on - off a few time but drops back to about 40-45 when you try to turn it over.
Wouldn't start tonight at the store where I bought the pressure gage on the way home from work, got to try it there in the parking lot. In the morning have to let the garage have a try

Not enough fuel pressure!!!! Fuel pressure needs to be between 60-66 psi for the engine to start and run.

Well 1 complete new pump/sender unit later, and a replacement gas-tank (original one rotted on top, replacement one a used oem) and it starts like it should. Gage says 62-65 psi when the key is turned on.
I have to take it back into the garage though, looks like they forgot to tighten a clamp or something, filled up last night and when it reached the top it started pouring out (probably the filler neck clamp) hopefully it also explains why it's used 1/4 tank of gas in 112 kms
Thanks guys