Think you could be burning wood at the wrong temperature and causing problems in your stove liner?
Try buying a stove thermometer and putting it on your stove pipe. I like the Stovax one, you can buy it on amazon for aound £15.00. The paint markings on this one tend to last a long time and the three operating temperature zones are clearly marked. If your stove burns to low in temperature then you can fill the liner with tar or if you burn to high you can open up the liner. Its magnetic, you just clamp it onto the stove pipe like a fridge magnet. If the stove pipe is stainless steel then it will not be maganetic in which case the thermometer comes with a wire which you can use to hold it onto the pipe with.