stihl 017 chainsaw?

Hi, my chainsaw starts and runs perfectly but after around 15 mins use, it just stops and will not start again. it seems to have the same compression and is not seized. When I go back to it the next day, it once again starts and runs perfectly for around 15 mins again. Anyone got any ideas.

ps I am using 50 to 1 mix and it is an older model 017; does anybody know if that type should be run on 25 to 1.
Thanks Steve