I have a 57 Chevy pickup. Straight six 235 stock engine. It should be as easy and basic as you can get. But I’m having trouble. When driving around town, no problem. But at highway speeds, especially going uphill, it starts to stutter and stall. I first assumed fuel filter. I changed it. Still had a problem so I changed out the fuel pump. Drove around town great. Drove it about 20 miles down the highway and everything was great…then….sputter. I got it home okay, but I’m not really sure what else it might be. I’m pretty certain it’s a fuel issue, but where/what? Suggestions?