1997 312 Fastech twin 454mag mpi's I get a vapor lock out of the port engine once and awhile. It runs about 8-10deg warmer than the other motor.. Seems these 454 and 502 were prone to do this.... Any thoughts on a fix???
Could it be my thermostat or seawater pump....? hell maybe all the above...

If you're running ethanol....stop. Port engine does it easier because the fuel filter is right between both engines and gets hotter.
The other solution and real fix is to get the Merc booster pump kits.

where can I get this... I want to fix it just once ya know.

Any Merc dealer can get them for you. They are an electric booster pump that plugs into the existing wire harness.