i've had similar problems with fluctuating idle and stalling. generally happened once engine was heatsoaked.

air/fuel ratio at idle became to lean which makes the engine fluctuate, the air idle control valve would try to catch rpm but just made fluctuation worse.

richer air/fuel ratio at idle is what helped me with this problem.

I have 1000cc ID injectors 2.1L engine and when running 98 unlead fuel at idle. operating tempertures. Injector pulse width is around 1ms and air/fuel ratio around 14-1. Also autronics ecu is in open loop.

ID 1000cc injectors minimum pulse width is 1ms. anything below the injector becomes unstabe or goes off its linear scale.