Engine stuttering issue

Hey guys,
I'm new here. So I apologize for the vagueness if ya
will. Anyways, I can do many replacements on
Chevy pickups minus serious engine/tranny
repairs.
Anyways, I just got this pick up truck a few weeks
ago and minus the fact I gotta replace a water
pump on it I have a serious concern and I am
hoping you all can help me narrow it down to an
exact problem or a couple possible problems.

It started startee two days ago mind ya, but it feels
like it is starving for gas. When parked or idle, the
tachometer pulsates between 400 and 700 RPMs,
as it tries to get gas. The check engine light
obviously comes on and the only way to make her
responsive is the keep pumpin the gas pedal and
after an initial couple pumps of nothing, she finally
responds and revs up accordingly and then she ia
fine for a bit. I can still hear her sounding a little
starved but she holds her own. I also hear a rattling
below when this is going on. And lately even when
accelerating slowly I hear here puttering as she
increases in foward momentum. I know for a fact
she needs a motor mount but I highly doubt that is
the problem.
Bad fuel injector? Fuel pump? Need to check the
pressure at the filter. Maybe a bad fuel filter? Or
maybe a bad sensor? Or is it something more
major like a clogged catalytic filter? I doubt its a
water pumo. Even though she just dumped all my
water, but who knows. Anyone else had this issue?
I guess it could be bad gas? Didnt do this until
after I filled her up. Im gonna put some Seafoam in
the tank and do an oil change for good measure.
She has a 4.3L Vortec engine. I guess I should
hook her up to a check system computer and find
out what code fires off but can't do that until I get a
new water pump installed hopefully tonight.