90 740 16valve 201K

I took a road trip up to Colorado a few weeks ago. I was driving up a steep gravel road and it stumbled for a few seconds and then died. It was very hot outside and the car had been pulling that hill, OK mountain, for a while. When I cranked it, it would stumble and try, but I couldn't get enough power to put it in Drive. I let it sit for 15 minutes and it started right up. I started back down the mountain and it died a few more times and after cooling off for 5-10 minutes each time it would run for a while. It continued to die randomly on the drive home. When it got cool outside it didn't have any problems. I also noticed that it did better in lower elevations -It quit at about 10,000 ft. It will always run OK if it has been sitting overnight. Then I can drive it for an hour or 2 before it quits. I haven't had my fuel pressure checked yet or read my FI errors. I replaced my main fuel pump under the car about 40K ago with a crappy autozone pump, (that's all I could get at the time). The in-tank pump I know hasn't been replaced in the last 80K and before that I've no idea.

A few people have said vapor lock, which I guess is likely, but it shouldn't ever do that, so what would cause vapor lock?.

whatcha recommend?

Thanks
- Nate
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90 745 16v 190k.