1st..... am now on the ground safe & sound, no damage normal landing.    The story goes....... Level at 2500' above Mason Jewett Field,  (  hazy evening, aprox. 7:30 pm  )  on the down wind abeam of the end of the runway,   4600 rpm,  ? MAP,  125 MPH  ind.,  3rd circle around the patch getting ready to put in full cruise pitch on prop to do a full throttle numbers gathering.  Then the eng. started running very poorly,  missing, stumbling hard, destict loss of power,  tryed to descend & come around  to land but am high & hot with  (  RV's need air brakes )  eng. smoothed out, chose a sweet spot & climbed out to go around,,, low  but climbing,  ran smooth 4000 rpm,  landed ok.  Before shutdown,  checked fuel map bars for EC2  on EM2,  some in the middle some at the bottom,,,Lean,,, why did the fuel map scramble suddenly,  steady cruise flight, made no changes at that time?  At the airport by 6:30,  pull plane from hanger to do two tests tonight, 1st. run eng. at low power/idle like sitting in line for takeoff from Osh or ?,  how long can we stay running without getting to hot, 18 min. coolant 205 F,  oil 175 F,  ok that's good enough for me lets go fly & do the prop full cruise pitch test run.  That was the start of the evening, ended differently than I would have liked.  But nothing bent.   Did not reset EC2  yet, am going to start eng. & check to see if computer B runs ok, did not get a chance to throw that switch, when I found the sweet spot I changed nothing, just came around & landed.  What is the fix for this anomaly??                    David R Cook  RV6A  Rotary