This likely has nothing to do with the problem, but you never know. I can see if someone leaves their fuel jug open a lot, that the voltatiles (methanol) would tend to evaporate faster than it would abosrt moister out of the air. so the old bad fuel would have less methanol in it and some additional water in it. so the percentages of the fuel components would be off then.
I just remembered that someone had run some tests on glow fuel contaminated with different percentages of water. He had some information about the effects of temperature on it too. if I remember he could run the fuel with a considerable amount of water in the fuel, up to a certain point though.
They were discussing in this thread here about glow fuel with upwards of 3% water in it would still work Ok. Someone claimed to have run 20% water in their glow fuel, but it didn't make sense to me at the time.
http://www.rcuniverse.com/forum/m_12...tm.htm#1306444