96 Chevy K3500 454 Runs really rough on startup sometimes, owner scratching his head

I've had my 96 K3500 with a 454 7.4L engine for a few months and ever since the weather cooled down from the hot summer days it's hit or miss
everytime I try to start it.

Now when I try to start it, it starts up running very rough about 70% of the time. It usually stays running like this for 15-20 min and mysteriously it runs normally again. When it's running rough, the rpms
head down from the 1500 or so to about 600 and bounce back up to 1500 back and forth until it heats up. It runs fine otherwise and it's been
relatively well maintained.
I've been throwing parts and work at it for a while hoping to catch the
problem but so far no luck. It has a full tuneup done with plugs, wires, cap & rotor, changed Idle Valve, changed upper and lower intake
manifold gaskets, and changed EGR valve. My next guess is that it might be fuel related, maybe an injector, fuel pressure regulator, or fuel pump relay??
Please help, I've been scratching my head about this problem for a while and I need to fix it before winter hits.
Thank you
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Landmanil wrote:

re torque manifold? can of ether with a straw trick should find it for you
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