2004 Sierra 4.8 stalling when approaching stops

The title says it. The truck starts fine and runs smooth as glass. It sat for about a year, then started right up with no problem and ran about 4 tanks of fuel through it. One day, out of the blue, it started stalling as I would approach stops. Once it stalls, I can turn the key and it will start right back up without issue, but will continue stalling if I don't keep the RPMs up.
It has thrown a catalytic converter code a couple of times, but currently there are no codes.

Has anybody dealt with this and know the answer?