try and stay with 10 %,15% would be better but also more expensive. engine is easier to adjust with higher nitro
as for the plug if the element inside seems to be black and full of carbon chuck it try to find enya #3 plugs best plug arround
or an os#8 would be o.k.
if your plug is used ,,,,full of black stuff or if the wire inside looks like its getting thick when you remove the glow lighter the plug cant stay hot enough so engine will quit.

did you touch anny adjustment (needles) when you switched fuel
and engine started to quit.if not than by going back to 10% and a new plug things should be alot better.if you dont want to wayst your 5% buy a 15% and mix both togeter,,,you will have 10% .
also it would be a good idea to add klotz oil to every gallon.
i use cool power fuel and oil content is 17% my ys engine recomends not to use under 21%oil content .1 1/4 oz of oil will add 1% to a gallon ,i think the tt 39 is at 20 or 21 % as well check your engine manual to make sure.

hope this helps

steff