Also make sure you're not filling the oil level too high. The crank frothing the oil could cause poor oil return from the heads as well. Do you have an oil temp guage? If so, how hot is the oil getting? Texas crude or synthetic? Viscocity? How long are you holding highest rpm? Your oil temp is probably the biggest question that needs to be addressed if this is a new problem and you're not running the boat any different from when you didn't have this problem.
Nick