My boat pulls very hard to left when reaching plane causing me to loose control. What would cause this?

I have a 1978 Mark Twain 18' with 302 Mercruiser. Took it out today and when I sped up, almost to plane, it would cut extremely hard to the left causing me to loose control of the boat. After I cut back power, it would not respond to the steering wheel for a short time, but at slow or idle speeds responded fine. I have an idea it may be the hydraulics, but how do I check this out and run down the problem?