Hi!
I have never in my 27 years of R/C flying seen or heard of a bad OS engine......only heard of ones ...from you in the states....

First of all ...Try to eliminate every possible fault, take away everything that might causing you trouble ...like the GP fueler ...I never use these things myself ! It is much safer ..and better to have just two fuellines hanging down under the engine ...one for fueling and the other for overflow...both closed by a short 3mm bolt!
Second what fuel do you use??
Over here (Sweden )most guys blend there ovw fuel...that way you know what you get....
Use 5-15% nitro and 20% castor oil ..that's the old stand by for nearly every engine on the market! Better yet is all the syntetic oil lfrom Graupner in Germany : Aerosynt 2 and Aerosave or the French brand : Motul "micro" - these oils are videly used in Europe in 10%-15% blends.
As for glowplugs : The best plugs for general sport flying in twostrokes are is Enya 3 or OS 8.....!!! Of coures there are others but these will make most engines work perfectly ...all the time.
What plug do you use?
Propeller : 14x5-14x6 for general sport flying at sea level!
Tank size: 14-16 OZ
Yes it could be that your tank is too high that is causing you trouble?? Why do you have it mounted so high????
What tank (brand) do you have?

Regards!
Jan K
Sweden