been meaning to ask this for months then AJ's post reminded me

ERST, pretty much standard.
starts on the button, but after about 2 mins of idling it will slowly drop in revs until it stalls.
you can keep it alive with lots of throttle then after about 2 mins of fighting it will idle bang on.

whats causing this? have tried cold start valve in and out, neither makes the slightest difference.