Evaporation
 

My field box is made so the fuel bottle sits in the middle of it and has the lines to the pump ran in through a special cap I made and the pump is bolted to the side of the box. Then a coiled line goes to the plane.

It is 15% fuel, and runs fine, but after sitting for about a month and a half, my engine doesn't seem to turn the RPMs it used to and doesn't start quite as easily as it did last time I ran it.

Could it be:

A: The nitromethane evaporated out of solution, reducing the content to 0%

B: Water condensed inside the bottle.

C: The methanol evaporated a little, raising the oil percentage.

D: My junky old LA is trying to lay down on me.

E: Just my imagination.

Evaporation
 

The easiest way to verify if the fuel is good is to buy a fresh bottle. You could also check out the glow plug. Another thing may be the mixture settings. Chances are that it is a little colder where you live at this time of year.

Anybody else want to throw in there .02?

Evaporation
 

If the nitro or methanol can evaporate then it's a dead certainty that water can get into the fuel. The can and all the lines have to be completely air tight when it's not in use. That means blanks on all the lines when not actually being used.
I'd go along with the previous suggestion about trying some fresh fuel...