While motoring at the end of a day of fabulous sailing on the Chesapeake Bay last weekend, the tachometer reading on my Universal 5424 (23hp) remained at 2000 rpm, even though the actual engine speed decreased. After about 15 minutes, the tachometer reading slowed down to the actual engine speed. I then increased the engine speed to 2000 rpm and the problem didn't recur. Any ideas about what happened?