I just noticed this question. I have both the Blade 500 3D (flybar) and Blade 500 X (flybarless). Both fly extremely well and are my "go to" helis if I just want to throw something up in the air. A couple of things for you to try:
1. Verify that your control linkage is in good shape with no slop/play.
2. Make sure that all bolts on the head are properly tightened.
3. Are your head dampers in good shape?
4. Make sure that your spindle (feathering shaft) is straight. A bent spindle will cause all sorts of flight issues.
5. Are your servos in good shape.
6. Use some expo on your airleron and elevator settings. 10% in normal and 20% in idle/stunt might be good starting points.
No heli stays in one place all by itself. But you can certainly tame down how it responds to corrections/control inputs. It sounds like your heli is overly touchy. Correcting that issue will be a combination of mechanical and radio setup.
Gaui 200, 255; T-Rex 250, 450 SE & SA, Mini-Titan, Blade 450