[cdnus]

sounds exactly like mine was behaving.
Fuel pressure dropping to "0" after warming up & driving for a while, engine idled like sh-t after warming up, front 4 plugs rich, rear 4 plugs lean.
After many hours changing out parts I now am 98% happy.
Solution was simple in the end.
Liquid filled guage was heat sinking & building internal pressure greater than pump pressure therfeore pressure reading would drop from 7lb. cold to 0lb. hot. Changed out to a non liquid guage. Problem solved.
As hood clearance issues prevent me from using a thick carb insulator I made up as large a heat diffuser plate as possible for the carb & found that by adjusting the throttle plate balance & idle air screws just right on my Pro-Systems carb I have fairly consistant plug colors front to rear.
To check if your problem is the guage try this, run the engine untill the pressure drops to 0 lbs. shut the engine off, apply a ice pack to the guage for a minute or so cooling the guage, if it climbs you have your answer.
I am amazed at the huge difference 1/8 turn on the idle air scews can make in my engine, not just at idle but low rpm driving.
I run a A/F meter so I can see the suttle changes.

Hope this is some help as I know the frustration you are feeling.

Craig