Well, assuming you know the basic techniques of both strokes, this type of problem on the FH side is generally related to either not watching the ball completely into the hit, grip changes or your talking the ball at different positions. Its amazing how just taking the ball a little different after the bounce will dramatically effect the flight of the ball. However, the first thing I'd suggest is make sure your really following the ball into the racquet - for instance see if you can see seams or trademark colors.
Lack of backhand power is often related to leaning back or not having much, if any, weight transfer. Check to see if your back on your heels when you hit the backhand.