1992 Chevy Astro 92 Astro Van ignition system

I have a 92 Astro van with a 4.3 non Vortec that runs fine when the engine is cold but when the engine is warm it will idle fine and when you push the gas pedel to the floor quickly it will rev up ok. My problem is when I press the gas pedel slowly the engine will stall. I pulled the coil wire off the dist cap and my spark is yellow. Any advice will be greatly appreciated.