From what I've seen around moving the thick part forward should not have THAT much effect, if any. But you say you used a much thicker airfoil as well as moving the high point forward. There may be other issues at work here. Can you offer up a pic of the airfoil section? And the airplane as a whole? Or at least what is your stabilizer area to wing area ratio?

If your shape is extreme enough you may have large separation bubbles even in level flight. With airfoil software and brand name shapes so available these days on the 'net you may want to give up on your own shapes and just use what's available and tested.

For what you're doing here, for example, I'd recomend the Eppler 474. Just pinch the trailing edge off a bit early to form a flar portion to join the ailerons to. At least you would have a proper leading edge shape that offers decent performance.