Hi,just need some help,I have a 1989 20hp mercury outboard,always starts first go,the other day started first go then went into forward throttle and all it did was just go at idle speed,after about 1km it just took off as if there was nothing wrong,but when i went to slow down it just would not get out of slow idle until I went up the river for a fair while and then came good,I cannot figure out the problem always been serviced and looked after