overheating/pressure control valve

I have an intermittent problem with my 2001 model z150txrz motor overheating. Sometimes I can go crusiing speed for 5 miles and there is no problem while other times i can go 5 minutes and it will overheat. I have only had this boat for about 1 year and purchased it used but with low hours. I have replaced both thermostats and the water pump. I was wondering what could be the possible causes of this problem. I was told by the Yamaha factory that a very possible cause of this malfunction was due to the pressure control valve. While looking into the motor I realized that this valve is quite far down into the assembly. If this is found to be the problem how would I go about removing this piece. Thanks for any help.