Had a similar problem while driving a 340 73 RR I bought in Albuquerque and drove back to CA. The car wouldn't go more than 60 and losing power. I stopped and changed the fuel pump on the side of the road. Then, I thought it was vapor lock, the car was sputtering and died in Winslow. Got it going again and it happened again. Barely made to Flagstaff. A mechanic looked at and couldn't figure it out. I looked at it again and discovered cracked vacuum hoses. I changed the vacuum hoses and drove the rest of the way problem free at 85 mph. The RR has a Quadrajet carb.
