Get another beer, this is a long one.
1997 Chevy Express 4.3 After
No codes
Short and long term fuel are fine
Cat in 187 out 229
Fuel pressure 52 lbs. (new fuel pump and filter) Fuel pressure holds for 10min after shutting off
Running around town it does fine.
BUT If I drive down the interstate 65-70 Mph (I'm an old man) for an hour or more. If I try to go above about 1/2 throttle it bucks and shakes, less than 1/2 throttle it runs fine. If I stop for half an hour or more it runs fine again.
If I let it sit in the driveway for 2 hours at idle I can't get it to act up. If I'm on the road by the time I get the dog box off to check fuel pressure the problem goes away.
What do I check next?

You may need to drive around with the dog house removed or get a long hose for the pressure gauge.
You need to be able to do some basic tests.

How many miles on it?

How many miles on the fuel filter?

How many miles on the spark plugs?

How many miles on the spark plug wires?

My guess is a fuel issue, but the others above could be related.

. . . and yeah also what carfixer007 said ^^^^^^^

Fuel pressure.

If you can drive with the doghouse slid back a couple of inches you might be able to have a fuel pressure gauge connected and visible while not also getting cooked.

What I need is a data log. Kinda hard to justify a $1,000 scanner for a $1500 van.

220 k on the van
150 on the fuel filter
Not sure about plugs and wires but it runs fine for an hour of driving. Even after that below half throttle

Here is an interesting read: https://chevytrucks.org/chevy-4300-v...gine-problems/

I take that to mean 150k on the fuel filter. Change the filter.

Separately from this issue, if you don't know how many miles are on the plugs and wires, it'* worth an inspection and may as well change plugs while you're there.

Your right that is interesting. Thank You

No 150 miles on filter. I replaced it when I did the third fuel pump.