My 68 dart with a mild 318 has a stock tank with a 1/2 in. line sending unit ,3/8 & number 6 fuel line tank to carb, running a carter, race mechanical pump, quick fuel 680 vac. sec. with guage on dual feed line. The Problem is ,when I fisrt start it in the morning it is unregulated at 9-9 1/2 psi. after running for 20 mins in traffic it is down to 5 to 5.5 psi ????? Pump gas premium, insulation around any hot exhaust with lines. Bowls are always half to 3/4 full. no vapor lock . Any ideas ????