looking for help or ideas, having problems with a nps40 that is running lazy.three years never had this problem. seems okay when first strarting but then gets lazy in 10 minutes after getting up to temp. cant leave on more than 2 hrs without burn pot filling up. have cleaned stove every which way i can.,been using cubex pellets which have been fine ,have tried from 4 differnt tons of these and still the same thing.have done the dollar bill test and it is tight, have resealed ash pan and still same problem. called company and they say i have an air leak somewhere. help any others had the problem?