My 2 cents
I have had the exact same problem. Startup is great @40psi then drops to <10 after warmup. I tried adjusting but could not get it above 10 when warm. I had this problem for 3-4 seasons. I finally bought a new spring/oil pin set from Moyer and had the same issue. I was however, easily able to increase warm pressure >20PSI which delivered ~60 when cold. True, the big drop between cold/warm may be cause for alarm but so far my A4 still runs great.
My suggestion is get a new spring and pin and adjust to 60PSI cold/20psi warm. Then run her till...whenever. I would not rush into a rebuild unless your a passagemaker.