tein,
If you are having the same issue i had with my 640, It had nothing to do with alignment. My situation was you would have the wheel at 9:00 to keep it going straight, even after it was up on plane. A little research and it seems that the reason it is doing this was the motor is trimmed to far under the boat, causing it to plow through the water. I have since pinned the motor on the second last hole from the bottom...so it cant trim all the way down. The last time i was out it was 45 miles an hour staright as could be....perfect.
Just a thought,
Scott