If you can,replace fuel line with a clear hose by carb. to see if you are getting any air bubbles. Then check temperature of fuel line from carb. to tank. It may be vapor locking. My 83' Chev. truck started doing the same thing and would even quit when pulling heavy loads. Found air bubbles in fuel line, inline pressure gauge showed after 20 minutes of idling pressure dropped to 2 psi. Replaced fuel pump, installed electric pump, no change. Replaced all rubber hose, caps, checked pickup tubes in tanks, no change. Checked temperature of lines and found 236 to 276 degrees between headers and tranfer case. Installed thermo sleeve on lines on the frame rail, mounted away from frame with nylon washers and built sheilds from engine cross member to transfer case. No air bubbles, fuel pressure doesn't drop at all now. Only took about 8 months to figure out and solve. The only thing we can think of, is the good gas we get now boils at a lower point. I installed the 454, headers and exhuast about 16 years ago and just started having that problem a year ago. Hope this helps.
Cameron