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Have you watched timing with a timing light when this is happening? Sticking mech advance in the distributor? Also have you checked vacuum readings (idle) to see if they are different?

One other thing to check is fuel pressure AND volume. You could have a situation where your fuel tank sock is plugging. When the rust and scale and s**t comes off of the bottom of the tank after a half hour of driving and plugs it up to the point the pump cant suck it out of the tank with enough volume. This would cause the symptoms you describe. After you shut it off, some of this crap can fall off the sock and back to the bottom of the tank and for a half hour it might run ok. Just a thought. I had a '67 GTO with a 428 I built and it did this same thing. www.rockauto.com get a new tank and sender for cheaper than you'd pay to get the old one steamed and treated.

Spectra Premium part numbers for 69 Firebird:
GM32B Fuel tank with filler neck - $94.89 perfect fit, exactly like original you won't believe how nice it is.
one of the two will be applicable for your application (these are listed under Camaro although the tank lists it fits 69 camaro and 69 firebird)
FG104D 2 port sending unit and pickup assy with sock $76.79
FG104C 1 port sending unit and pickup assy with sock, 3/8" line $76.79

They had a ad in Hot Rod magazine with a promotional code that gives you 5% off of that too that runs through the end of this month, enter hotrodrewards in the "how did you hear about us" thing during the checkout process.

Hope this helps,

Mike

BTW, your temp readings look very good, no hot spotting there.

The official way would be to put a T in the fuel line going from the tank to the pump and connect a vacuum gauge. Max allowable restriction is 2" mercury. You could also use that gauge and check pressure, which would show a considerable pressure drop at higher load.

Or, try this. REMOVE THE FUEL CAP. Take the line off the fuel pump and try blowing back through. You should be able to hear bubbles from the fuel tank without much effort at all. If it feels like you're trying to inflate a tire there's a problem. Again with the fuel cap off, you could try blowing very regulated compressed air backward down the line to try to temporarily blow some crud off the sock. More than say 20 psi and you'll blow a hole in the sock though. Use caution. (Also note I have had little success with this over the years.)

Or see if you know an electrician or mechanic with a borescope, a really handy tool to snake down the filler neck and peek around inside the tank.