This is one I really did not want to post on. It could be the injection pump. You are not running wvo or are you?

Most of the posts that the owners suspect it may be never usually are. Things I would look for first.

Fuel pressure is adaquate and sustained in the base of the injection pump when returning to idle. Takes a meter to verify. A rough guess would be if the relief valve is constantly overflowing but then you have to assume the valve is functioning properly. Good place to look for air ingressing into the system earlier as well. The relief overflow line should only be releasing fuel.

If those things check out well I would internally soak the injection pump next. Using a good solvent as some crud may be interfering with the govenor.
Again it is semi important to mention if you are using wvo or not. I would also check for water in the fuel tank before going very far. This is unlikely but something unusual is going on. As mentioned earlier air in the fuel can do strange things as well so it may be the best place to start.

One other last thought was if the shutoff on the injection pump might be contributing but perhaps you could tie it open temporarily to eliminate it being partially activated somehow. Or remove the vacuum line from it. Another long shot was if the engine was dropping a cylinder when it drops to idle and picks it up again. I assume the valves clearances have been checked reciently. The hot valve stem would tend to start to cool and shrink as soon as you were at idle again. This might enable the valve to seal well enough to let the cylinder come back on line again.