I have a 68 Chevelle SS with a 66 corvette 427 motor with GM 502 aluminum performance heads .030 over bore, speed pro pistons and 1.7 roller aluminum rockers. I recently put in a 533/533 lift-313/313 duration cam and had to do a lot of fine tuning with timing, rejetting my holley 800 dual feed and internal distributor work. Electrical now is great. Car runs strong. I also installed a new mini high torq starter, new battery, recurved the distributor and put in lighter springs and a new holley 80 gpm mechanical fuel pump and new rubber on the gas line. When I start the car fuel pressure is around 9 lbs. After idleing or running car around 10 minutes fuel pressure start to drop to 3 or 4 lbs. I have a glass fuel inline filter and it shows about half filled, no regulator just a pressure guage. I was told maybe the pickup sock in the gas tank is blocked or maybe I have a bad fuel pump that when it gets hot the spring is weak. Any ideas.