1987 135 V6 showing hot on the gauge

I have a problem with my v6 135 Mercury It is a 1987 model. I have replaced the water pump, the thermostats and poppet valve. If you run the motor over 3000 RPMs The gauge will move to hot quickly. Then the alarm sounds. The head temp shows 190 I now have removed the heads to replace head gaskets. Everything looks good. The heads are flat no visable cracks. The temp sensor checked good. Can any body give me some advice?
Thanks for your help.

If this engine has been run in saltwater mostly you might find this helpful as it happened to my new 1999 yamaha 150 motor I got a few months ago. Ran fine until I got over 35mph ny gps. Waterpump replaced all else fine. Removed cooling water covers on cyl heads and sprayed vinegar/water mix on all the cylinder casing where cooling water runs ( behind covers I removed). Then scraped all the built up scale off. Replaced gaskets, never overheated since!

Thanks for your help. This motor has not been used in salt water. I replaced head gaskits, water pump. thermostats and poppit valve. The motor ran fine till about 3000 RPM then the temperature would go up in a short amount of time. I thought about what the poppit valve does. I removed the poppit valve. Problem solved. I know this is not correct but the boat is leaving on a 4 day fising trip on Sunday (without me). I had rather it run cool than destroying the power head. I will figure out the valve problem when it gets home. I have owned this boat and motor for 21 years. Other than normal Maintance I have replaced one stator the entire time I have owned this Rig. You cannot complain about that kind of sevice.
Thanks foe your help.