Well if it makes you feel any better (and I am sure it won't!), I had the same issue with my 1/2A Norvels. As the element would "frost", you would need to lean the engine out to get an ok run (of course you didn't know why this was going on). As you would fly, the poor Norvel engine would begin to overheat, finally sagging down to a miserable (but long) run at ~1/2 power.

Only solution (except for one below) I had was to change plugs.

Norvel claimed that their engines ran cooler than others (presumably meaning Cox) and this cause "stuff" to plate out on the glow element. Certain fuels were alleged to work better--not having this "impurity". As I recall Cool Power and Omega were two of the ok fuels. They eventually came out with their own Norvel fuel, and I know guys who swear by it as working, and I have to agree that they don't seem to have the problem I had. I know other guys who use Sig fuel and also don't seem to have issues. I just know that I did and still do (I use Sig). I actually think Sig brewed the Norvel Fuel, so go figure.

My final solution was to go electric!