That definitly sounds like you are overheating. Try adding a touch more 2-stroke when you mix your gas and richen your mixture a few clicks. Also check your spark plug. If it has white char then you are well on your way to an engine seizure.
P.S. If you do add more oil then make sure you carry an extra plug and plug wrench in case you carbon foul it.
Hope I could be of some help.
Jon Lewis