2005 Chevy 1500 4.3l. Running rough

I am working one of my company's trucks, a 2005 Silverado
1500 with a 4.3. At first it was running very rough. It would
fire up fairly easily, but didn't want to idle. I'd give it has and
it felt like it was running on 3 cylinders. I hooked up my
OBD2 tool and it gave me the generic 0300 random misfire
code. The first thing I did was pull the spark plugs. Most of
the insulators and electrodes were black so I changed them
all out. This helped some but it still doesn't want to idle. If I
held the gas pedal down and kept it at 1000 rpms its
smooth as glass. Let off and it stumbles,  almost dies and
settles on 750 rpms. Turn on the A/C and it dies. I put a
brick on the gas pedal to give me a steady rev at 2000 rpms
and listened for a vacuum leak. I found one at the brake
booster. A grommet was split at a check valve. I replaced it
and no change. I unplugged the Idle Air Control valve and it
did nothing. So I thought maybe its bad. I plugged it back in
and it smoothed out! But it still stumbles when I let off the
gas. Still dies when I cut on the A/C. I unplugged the TPS
and it didn't change anything. Does that mean the TPS is
bad? Any ideas on where to go now?