Terry, what Oil are you using, viscosity can change dramatically with some oils when an engine gets hot and then loss of pressure can occur. I have used Fuchs 5-40w Pro-Race Ester Fully Synthetic in my 4l +8 and this was very stable across the heat range and in fact the engine ran at about 5 degrees cooler, however I was advised to use a 20-50w Synthetic as makes quieter running when hot, and now use Valvoline 20-50 fully synthetic, pressure is up a bit but not as good on temperature. If you can find a fully ester synthetic 20-50 then you will get good results.
You say your engine is new, so no pressure at hot tick over is odd, sounds like the gauge, and/oil combo.
Let me know how you get on.
