What size is the mounting stud for the head? Stihl makes some excellent bump heads in 5/8" 16 RH thread.
But something I've learned the hard way: Replace all your trimmer line yearly! I mean it! Throw away whatever is left from last year, including the line in the head. Buy some new stuff! Yeah, it hurts--I'm as cheap as the next guy--but the frustration of constantly breaking line hurts more.
I just bought a new Stihl professional-grade 36V electric trimmer. I almost threw the bump head that came with it away as the line kept breaking inside the head. Before I tossed it, I decided to change out the line it came with... no more breakage problems.
I just bought this, based on advice from a friend who owns a landscaping business:
It's the best I've ever used. It tends to bog the trimmer down before breaking--even when hitting cyclone fencing.