Hello Henryw and welcome to FSM
My compressor has a small tank on it therefore the drop in pressure is far less excessive than what is happening with yours.
What type of compressor are you using ?
An 80 - 20 thinner to paint ratio (if I am reading that correct) is way to much thinner compared to paint IMHO. It should be the other way around. Some paints, such as Tamiya acrylics, I use straight from the bottle without thinning.
What type of paint are you using?
Happy Modelling and God Bless
Robert