Every time you cut anything with it, before going back into another piece of wood or if you are cutting a large tree half way through it also pull out, then rev it wide open three or for times to get more oil on the bar and chain.
Now a friend of mine had to take his to the Stihl dealer and have a new filter and line put on his and solved the problem. It appeared from what the dealer showed us when he picked up the saw, but when he was filling up the oil res there was debris falling into the tank thus stopping up some of the filter.