Stalling/poor idle

1994 Chevy 1/2 ton silverado 4x4 350, approx 320,000kms.

The truck when idling low wants to stall, it runs well when moving but whenever you slow down or are at a low idle it wants to quit. We've already replaced a failed fuel pump and ignition module but the problem still exists. Since replacing those parts the truck hasn't stalled, but it wants to and it's running poorly at low rpm's. What could be causing this?