I would suggest that once the vehicle stalls, pull the spark plugs and see how they look. This may help direct you into what direction of diagnostics to follow.
- Light tan spark plug: Good mixture.
- Dark brown or Black Spark Plug: Too rich
- Black, oily Spark Plug: Oil fouled (see valves or piston rings.)
- White, no color on spark plug: Too lean.
- Good running conditions: If everything is good, the spark plug should have a tan/light brown color.
- Rich running conditions: If your engine is running too rich, the spark plug will be black and sooty.
- Lean running conditions: If your engine is running too lean, the spark plug will be white.
- If the spark plug is black and oily, they are oil fouled.