Any Merc. Mechanics onboard?
 

Have a 2000 502 MPI Temp gets up to 225 then falls to 130
as this happen I noticed my water pressure drops from 35 to 20. Checked my Sea pump and back flushed the system changed the thermostat there was tiny pebbles in the old one holding it open is what I thought would make it run cool. Any insight is appreciated. Live in an area that has no High Performance Mechs for boating.