94 Silverado Slow Crank, No Start

I have a 94 Silverado (GM 5.7l 350). The other day I
drove it home from work. She ran just fine. Went
inside, came back out 10 minutes later and she
gave me a really slow inconsistent crank and didn't
start. I had the battery tested, it tested good and
charged. So I came back home, put it back in and
tried again. Still had a really really slow
inconsistent crank, but she started this time.
Rumbled and ran real rough for a few minutes then
came out of it. I let it run for a bit. Shut her off, and
then tried to start her again....really slow crank no
start. So I pulled the starter, it tested fine but I
replaced it with a new one anyways. Installed the
new starter, got a real slow crank again, and then
she started. Same as before I let her run, shut her
off, and when I tried to start again, really really
slow crank, sometimes no crank, and no start yet.

Could the Alternator be the cause of this?
I am getting fuel in my carberatur, I can smell it and
I can hear my fuel pump activating, so I know it's
not fuel pressure. I checked my cables and wires,
everything appears fine. I'm stumped.