I Had the same problem for a while . It was because I had started lifting my arm up and to the side a bit. I was heaving the line instead of snapping the line forward. When I brought the rod in closer to my body and concentrated on snaping the rod to the stop position and stopping it abruptly the problem went away. It is a bit like a swinging a hammer and stopping abruptly as it hits the nail. Your rod should stop like that to impart power to the line. Also I realized that while false casting I had forgotten to stop the line in the proper position and was instead aiming it at the ground/water where the fly should land instead of aiming it above the ground/water during the false cast . it should be aimed at the ground/water at the end of the cast .
I hope that makes sense, it is quite hard to describe.
Darn with this new format I don't know how to place a link to anything so i have copied and pasted this from the Castwell articles by JC

HOW TO CAST

1. String-up fly rod, pull out about 25 feet of line.
2. Extend left arm in front, face high.
3. Bend left hand at wrist, palm facing you; keep it there.
4. Swing fly rod and line behind you.
5. Swing fly rod to front sharply striking palm of extended left hand.
6. Notice, as rod stops at hand, the line shoots forward; Duh!
7. Repeat step #5 two more times.
8. Notice,that the harder rod hits hand, better fly line goes; Duh!
9. Notice left hand now hurts. (If not, repeat #5; this time, harder.)
10. Repeat step #5 WITHOUT HITTING HAND. (Just STOP rod in same place.)
11. Repeat step #10 two more times.
12. Swing fly rod to rear stopping in same way as #10.
13. Repeat step #12 two more times.
14. Combine steps #10 and #12.
15. Repeat step #14 two more times.
16. Call buddy; have him come over and teach him how to cast. ~ JC