02 yamaha 115 v4 2 stroke runs then RPMs jump, water pressures falls

I have a 2002 yamaha 115 V4 2 stroke that starts and idles fine. i was out today and it was running fine at 3000 RPMs then the RPMs jumped to around 4400 without touching the throttle. As the RPMs jumped the water pressure gauge fell from 20psi down to between 0-5psi. i backed off the throttle to neutral, waited a second and ran back up to 3000 RPMs without a problem for afew minutes then RPMs jumped again. Got the boat back to the dock and changed the spark plugs as they were due. One plug as alittle dirty but nothing that i would expect would cause a problem. I ran the boat again and had the same problem. Any help would be greatly appreciated.

You did not give a lot of exact details so here goes ----------------What is the model # info of the motor and how does that compare to your transom height.-----------------Could have the wrong set-up.

Ive owned the boat for almost three years and never had an issue. I am the second owner of the boat and have always kept up on the maintainance. Motor is mounted on a 6" jack plate. i was running the jack plate all the way down when it first started. I tried differnt heights after the ploblem continued. Motor model is a 115tlrb.