rough engine/power loss

[Donald A.. Russell]

Hi all,   Would appreciate some thoughts regarding the problem(s) described below.   The engine is an 0200.   Everything is ok on runup,mag check,carb heat,mixture checks and takeoff are all good.   About 15 minutes into the flight( over the field!) the engine loses two to three hundred RPM and becomes slightly rough. Advancing the throttle to regain lost RPMs does not work unless throttle is first closed then reopened. This works for a few minutes and then problem reoccurs. Applying carb heat smooths the  engine but drops RPM even more. We're in sunny FL.   So far we've done the following: Marvel Carb with two pc venturi checked by Approved Repair Station, # 2 and # 4 cylinders reworked, also by Approved R/S. Mags overhauled ,reworked with new points, condensers and timed to engine. Also App. R/S .Compressions are all very high and uniform.