"I have a 2003 volvo stern drive 4.3 gx with an SX drive. Can you help with my issue.

The boat has a slight vibration when in neutral running at 5000 rpm oil gauge looks normal engine sounds normal. When I put it into gear, it is slow to take off, the engine at full throttle only reachs 4000 rpm not 5000 rpm and reaches about 30 miles usually 45 there is a small vibration and the engine sounds louder through the exhaust like it is working hard. The oil guage bounces around once the vibration starts(I am thinking this is caused by the vibration, oil looks good I have only put a few hours and it is still clear and looks like new. The problem started when I started with the engine slightly up but not above what the owners manual says you can run the drive. once the problem started I drove the boat for 20 minutes back to shore. Shifting in forward and reverse are no problems at the dock was no problem the engine temperature was normal. There is no drive damage or prop damage from hiting anything.. Got any suggestions."