I bought a used bass tracker with a 1998 60hpmercury. Twice in about 6 trips when I shifted into neutral the motor did not rev down. It rev way up. I didn't want to shift it into gear witht the rpms that high but had to choice once I got back up on plane and ran it a few minutes when I shifted back into neutral everything was fine. Any ideas why the rpms would be so high those two times.