Hi Yves, in post 191, you mentioned having to use nearly all of the available adjustment of your valve clearance screws due to the pushrods being a bit short.
This would suggest that the angle of the rocker to valve may be out, could this have caused the valves to touch each other when in the overlap position and allow the pushrod to dislodge? Even if that isn't the case, it might be worth addressing the issue as there is a chance of increased guide ware with an incorrect rocker angle to valve relationship.
Just thinking out loud as it is always difficult to diagnose problems remotely.
You could check the clearance between the valves when they are closest together (usually around TDC) with a piece of bent wire through the plug hole, I think Jim Schmidt suggests a minimum of 40 thou clearance (I use a piece of welding rod filed to 40 thou, if it goes between the valves I'm good to go)