1993 14' boston whaler rage mod: 90 jeer sn:go3561784 in salt water. unit over heats after 4-5 min. o'yea i just bought the boat the jet drive is all new to me. the boat worked fine the first time out then it overheated then ok then overheated again.i checked the top of the heads for flow both equal at an idle it took about 10-15 sec to get water. removed thermostats and checked them they open 135-140. what can i try next 