I'll be interested in any suggestions as well - since my 912 does the exact same thing.
However Im on Webers40IDFs and my first suspicion was the distributor sticking/not settling back fully after advance? If I adjusted and setup my carbs at say, 10 degrees BTDC and then (after idling sometime) the distributor settles back to 5 BTDC - wouldn't I get these symptoms? When adjusting to a higher idle there is more centrifugal force in the weights and the drop would not occur? It's a theory anyway...?
How difficult is it to get to the weights on a Bosch .031 - Anyone have a link with pictures?
Thanks!