I had a liquid gauge under the cowl and it would drop when it got hot also. What I found out was it would automatically change based on barometric pressure. I changed it out to a non liquid gauge and problem was solved.
I had a liquid gauge under the cowl and it would drop when it got hot also. What I found out was it would automatically change based on barometric pressure. I changed it out to a non liquid gauge and problem was solved.Liquid filled gauge is my bet. Change that and I bet you'll find your problem.