Hi - I have a 1982 OMC 2.5L 4CYl with 400 series out drive. The boat has been running great all summer. Last two times I have been out, after running on the water for around 10 minutes and then I stop - as I take off again the boat starts to come up on plane then the RPMs jump way up like something is slipping. I throttle back and ease into it again - starts to go then slips - finally it gets up on plane and runs great. Happens about 3 times the entire day - always when I am taking off -(under load). The prop is not stripped. What else can I check? Any ideas?