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Greetings to everyone, and I ask the more experienced for advice. (1953 Packard Clipper with a 288 cu.i. engine.)
I have a problem with fuel shortage after the engine warms up. Everything is fine during startup, the engine runs as it should. However, with increasing miles, less and less gasoline gets into the carburetor. After 6-8 miles, practically no gasoline reaches the carburetor. The car jerks until the engine stalls. It takes about 30 minutes to cool down, then after starting and a few miles, the same issue repeats. Until recently, everything was fine. I haven't replaced anything lately, so it can't be caused by a new/repaired part.
I've noticed one more thing; I don't know if it's related. Apparently, due to high temperature, the spark plug on the first cylinder changed color. It's no longer shiny but dull and slightly blue. The rest of the spark plugs are fine, still shiny. What could be happening in the first cylinder?
Thank you for any advice or insight.

The plug wire boot may be allowing a spark to jump from the boot to the plug body. Dirt on the plug insulators can aggravate the problem. The spark plugs need to be clean and have good gaskets and the cylinder head where the plugs seat needs to be clean and free of rust. Seepage from the water outlet gasket on the head could discolor the plug.

Fuel supply issues from a variety of causes often get worse as things warm up. Fuel filters added to the suction side of the fuel system often give trouble.

A clogged fuel tank vent can cause the problem you are having. Any restriction on the suction side of the system, such as dirt, a dented fuel line, any screen or filter, but especially any added, non-original filters, are likely culprits. Missing heat shields, mis-routed fuel lines, stuck manifold heat valve, and leaking exhaust gaskets can cause problems.

A failing fuel pump, loose fuel pump to block fasteners, or missing heat insulating washers and sleeves at the fuel pump mounting can cause problems.

Late timing, leaking intake manifold gaskets, and carburetor issues can also contribute.

Good advice from TxGoat. You said 8 miles, so that's probably 15 min of the engine running? I would have to guess fuel tank ventilation. I believe 53's used a vented cap, but I don't remember specifically. I don't think it would hurt anything to buy a vented cap for testing.

Example:
Let's say your tank is 3/4 full (=5 gal air) and you use half a gallon of gas in driving. Disregarding fuel tank shrinkage, you have expanded the air by 10%. So instead of about 14 PSI absolute, you have 12½, or in other words -1½ PSI gauge pressure from the tank to the outside that the fuel pump would have to overcome. Considering that fuel pumps often only pump about 2-4 PSI, I could definitely see this being the issue.

Probably debris in the fuel tank. Remove gas tank sender and see if there is a filter of some type.
If so, replace it.

Also when driving you consuming fuel faster, so if the pump is marginal, you could be using fuel faster than the pump is able to replenish it.

My 53 Cavalier has a vented fuel cap, so I would guess yours should as well. Easy to check and easy to fix.

To me this does not seem to be a heat issue, this is a flow issue. If the fuel cap is venting, that is, it is not creating a vacuum and starving your fuel pump, then I would try sticking an air hose on the fuel line and blowing back to the tank, just to make sure there isn't a restriction in the line from the tank. Then, as mentioned, I would test the fuel pump to make sure it's not weak and just not keeping up once you start driving. Do you know anything about the condition of the fuel pump?

Your filter does not appear to be dirty, so I'm guessing your fuel tank is fairly clean.

Hopefully there is an ice cream shop within 6 miles so you can go for a treat, let the car recuperate, and then drive home.

Drive it with the fuel cap off. If the problem goes away then you have the wrong fuel cap (not vented).

What BigKev said. Had the same problem with my '37 when I swapped the (better looking) '40 cap. Stalled the car within two miles. Baffled me. Rebuilt the fuel pump. same thing. Pulled the cap.

Ran perfectly.

Make sure you let us know what the fix is!