Have an old stihl 029, almost 25 years old and it has seen a lot of service. Has always been rock solid til just recently. Since day one- full choke on, pull 8-10 times and motor pops - drop back to half choke and throttle it she jumps up to normal choke off and is ready to work. Saw will run for maybe 1/4 tank or little less and saw revs up like its about to run out of gas. Start playing the throttle and theres no saving it. She dies and will not restart until after sitting for 5 minutes or so. I have removed and cleaned air filter element, changed plug, changed fuel filter and shot carb cleaner thru carb. Acts like a fueling issue, hose from tank appears ok but I guess it could be collapsing. Can the fuel cap be causing this from a bad vent ? It doesn't seem like the fuel tank is building pressure.