You were taught to balance "a little tail heavy"? What the.......?

No way Jose! You should, at almost any skill level, balance a plane so that it's a little nose heavy. I try to do this on every plane that I'm just trying out and will move the CG to the rear as I get used to it.

I agree with Great-North. Check the obvious. Are you sure your engine is running right (fuel leak, running rich, etc.....), is the elevator reversed (been there done that!)?

Good luck,