Engine Temps?
 

Changed my thermo 140 for a 140, cleaned housing and front of intake cross overs with screw driver and shop vac. Now my Merc 260 will only reach 120 at all times except when tubing or skiing for extended periods of time. I am thinking of putting a 160 in to get the temp up to a decent range while cruising. It use to run about 150+.

Also, the weather out here in DC has been 95+ for over week and I seem to have lost about 400RPM. Is that normal on very hot summer days or could my lower engine temp also be aiding?

High ambient air temperature will cause a performance loss. Do a search on "density altitude" on the internet to learn more on this. As far as the water temps you're seeing, I like to know for sure that the gauge is correct, then make a thermostat change. As a side note, with higher density altitude the combustion chamber temps are lower. But I doubt that you'd see a 20-30 degree drop in coolant temps because of this.