First thing I would look at is my chain sprocket if it is worn you might want to replace it with a new 7 tooth/.325 rim sprockrt kit by Oregan off of e-bay for $14.99 + shipping. I like the rim sprocket better because the chain always rides in the same spot and therefor doesn't loosen up after you start using it.
Look at the front of your saw at the base( power head) end of the bar opposite side of clutch Sprocket cover there should be a flat head screw that is your chain tensioner if the screw aint there go get a tensioner