RE: Club FOX!
Here is my two cents worth of unvaluable advice.
If it slows down at full throttle when glow plug is removed you need:
a hotter plug (fox standard long is hottest fox), more compression (I do not run shims in mine with 0% nitro, the soda can shim likely lowered compression, try without any shims), more nitro (10% should plenty with a shim), too rich (lean until pinch test results in no increase then back off 1-3 clicks), poor compression. My 60s are eagle IV however.
Also that MAN article was for an Eagle IV, yours is an older version but it is still to far off.