high rpm-low speed

Question
I have a 1997 Regal 1700 SLR with a Volvo Penta 3.0 4 cylinder engine and a SX sterndrive.  I've owned the boat for a month now and was able to run is up to 35mph at 3500rpm  with two adults on board.  The past two times I've taken it out, I've only been able to hit 25mph even at 4400rpm.  I have been running a 14 1/2 x 19 prop.  I have not done anything to it mechanically.  The only repairs I've done to it have been new belts, new ignition and new engine kill switch.  Any ideas?

Answer
Hi Carlos...Thank you for selecting the undersigned to respond to your question.

Something is terribly wrong, and it sounds like it could involve the drive shaft. Although I'm not a mechanic, I recommend you consult a reliable and reputable marine mechanic.

Safe Boating
Ron