Echo PB9010T backpack blower loses power the longer it runs

Purchased a brand new Echo PB9010T blower about six weeks ago...ran great when I started using it. However, I have noticed as of late the following...

- Runs great at startup
- The longer I run it, the less power I have
- It seems to decrease in rpm's the longer I run it
- It seems to run rougher (sounds more noisy, not the smooth humming noise I should hear at max rpm's) the longer I run it
- Then....the next time I use it, it starts off working great and then starts the same decline in power and rpms along w/ sounding like it is running rougher
- This phenomenon seems to start becoming noticeable about 10 mins or so after starting

I purchased a new gasoline container specifically for this blower as I use either 91 or 93 octane gasoline from WalMart's Murphy Express and I mix it with Echo Red Armour 2-stroke engine oil at the rate of 2.6 ounces of oil per gallon of gasoline!

Any thoughts/suggestions as to what may be going on?