Chainsaw problem

I've had a Stihl MS 310 chainsaw for 6 or 7 years and it has always been fairly easy to start. However, I've had a problem starting it on a few occasions over the past year. Normally, I start it in the full choke position and it'll fire on about the 3rd pull. I can then move the control to high throttle position and re-pull and it will run - and, then move it to normal run position. Usually, the cylinder compression is so high, it's difficult to pull. The times that it won't start, the compression is apparently low because the crank cable pulls easily; almost like ther's an air leak somewhere. I've replaced the plug and air filter to no avail. But, the compression loss has me baffled. On the last start failure, I simply let it sit on a shelf for 3 weeks and then tried it again - it was back to it's old self and started up normally. Has anyone experienced this type of chainsaw problem and might you have any insight on what's causing the change in compression ?