John,
From your description it sounds a little like you have a plug problem. I have had good runs with a hot Enya#3 plug (I have also used the Fox plug work but not always, sometimes they get taters on them and don't hold the setting).
The Fox is a special engine and once you get the hang of running them they are a very good stunt motor for the right size ship, which the Ringmaster is just fine. Not knowing how much you have run stunt engines I will give a few basics (if you know this stuff please forgive me). Break the engine in slow on the Brodak 5%, 28% oil fuel, 9x6 prop, Enya# 3 plug (this is a good starting place, a Fox plug can work but may not). This takes about 10, 5 minute runs. On the last three runs do some lean-out bursts where you either lean the needle a bit at a time or pinch the fuel line to cause the engine to lean, holding this a little longer (20 sec) several times during the tank run. Once it will hold a lean run its ready.
OK, mount the engine on maple mounts, don't use hard Aluminum pads or if you do add 1/32" plywood pads under the mount lugs (to allow some crushing under to account for the mold draft angle of the mount lugs). Mount the tank so that the back end is kicked out about 1/8" and the centerline lined up with the upper head fin (the Fox likes the tank a little high and the first fin on the head is a good start point). Last, assuming you are using a uniflow tank set the vent up in the free air stream (see the attached photo).
For the first flight set the engine on the rich side so that if you pick the nose up to the 11 o'clock position it just breaks into a 2 cycle. Once you get a feel for how it runs you can bring up the rpm a few clicks at a time to get the lap time in the 4.9 sec (assuming 60' lines).
If you start doing maneuvers you will likely develop the Fox "Burp" on outsides. This is what they do and it can be eliminated by running more of the 2+-2_2+ setting or do the stick in the bypass (see attached).
Good luck, DennisT