Marine engine mystery
 

I have a 2013 Volvo Penta 350, 225 horse in a four winns boat. The engine was running fine. The next time I took it out, it acted like a car with the clutch slipping. The engine was revving but the boat wasn't going very fast. At 4000 rpm the boat would normally be at 37 mph. With the problem it runs at 30 mph. The boat when running normally at wide open throttle max's out at 5000 rpm. With the problem, it wants to rev way past 5000. I took to a repair shop. They ran the boat and it ran fine for them. The next 3 times I took the boat out it ran fine for me. I took the boat out yesterday and the problem was back again.

Any suggestions. Thank you.