01 GT-40 limp mode/lowRPM

I'm grateful if any body here can give advise to solve my GT-40 problem,
my boat had 2001 SAN w/GT-40 engine,before have higher temp. symptom, until I solve this problem,used for about one year at 180F,when RPM runing over 2600,the temp guage seems tend over 180F,so I need to lower the RPM to let temp lower,finally,I solve it by change a 147F thermosat,now temp reading 160-170F, now another problem happen,first time was happen before changing the thermostat, the RPM,limited under 2500, when next time use ,it became ok at morning,when later use the same day,it happen again, and can't recover ,now the max RPM only2100-2600,even full throttle, when running, I can feel the engine vibration little stronger,max speed only 18KMH , I check the coolant, I notice the level become higher but later become lower when stoped so I add about 500ml water, I notice the coolant will become less running sometime,I check all sparkplug,all is fine but in dark color,(high temp recult? ) I hope some body will give me advise.