I used to have this problem badly! any time it got cold I was scraping the inside... Got a pingi and that improved it greatly, cleaned out pollen filter and now it's a really rare occurrence. Having said that it's started to appear slowly, need to microwave my Penguin methinks... (oh and P.S don't be an idiot and run cold water over it straight after taking it out of the microwave, it broke the indicator thingy telling me when it needs recharging
)