Could also be his steering actuator going bad. Mercruser has pressure gauge testing and diagnostic procedures to follow along to pin point the problem. OEM Service manuals is your friend. They are a must if you want to work on your own apps.

Op
You asked for help, and a lot of people would like to.know the solution, please keep us in the loop!

^^^^Yup !!!!^^^^

Again, I got one doing it and I would love for someone else to figure it out. LOL.

I had this exact problem years ago and it was a piece of rubber chard looking material caught in the pump pressure relief valve. Only did it at wot and would pull hard right. It would go back to normal once slowed down again. This valve is the fitting that the hp line screws into on the back of the pump and is easy to inspect and clean out/
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Well, I put 6 hours on the boat last week and the problem continues. I would trim up at idle and I had no problems. At about 4600rpm and just over half trim the problem is reoccurring. I did notice I have about 1/8 a turn of no response (slop) in my steering wheel. It looks to me I need to take it in to be looked at. I think this is past my comfort level.
Thanks for all the advice.
I think the "trade in light" is starting to flicker too.