While out on the water today my 1996 700 SL started to make a horrendous noise at about quarter throttle and suddenly stopped running. I hit the starter and it reluctantly started up, but knocked above an idle. While attempting to nurse it to shore it suddenly started to run fine. I had about two miles to go and had no problem at any R.P.M. About an hour later I took it out again and it did the exact same thing. I feel that I have an above average grasp of the mechanical working of an engine, but am completely baffled by the behavior of this engine. Any suggestions would be appreciated