starboard motor? I would have someone put the boat in and out of gear and take a look in the strainer. I know on mine you can look through the plastic lid and if there is air you can see the bubbles. Normally when the motor is first started there is some air then it clears out. If you are getting that much air you will also hear it at the sea water pump. Weird problem. How is it when running the boat on plane? Have you double checked your guage to be sure it isn't a bad ground or sender? How is the water pressure? Let us know what you find and good luck.