The first thing that comes to mind is, what is the motor's shaft length? I had an older Grady Offshore with a 20" Sea Drive.... did the same thing.
If it's a 25 inch, (and it should be) is it mounted too low? Trim the engine straight down and look at where the anti-cavitation plate is, with respect to the keel. It should be around 1 to 1 1/2 inch's below the keel. If it's lower, you should be able to raise it, (check out the bolt pattern on the motor bracket).
Are you trimming the engine down? It's usually a good idea to trim all the way down when your getting up on plane. It helps to keep the bow down. You can trim out, once your up and running.
Last thing, do you have trim tabs? If you do, extend them fully before jumping on plane. They'll lift the hull and help to keep the boat level. You can back them off, after your on plane, (same as engine trim).
If you don't have them... get them! Imo, they're an absolute necessity.