Again, it's a 350ci/5.7l GM V8 with a carburettor and roughly 250HP and no electronic ignition system.
Thanks in advance
pj
Hydrostössel is most likely 'hydraulic valve lifter'. (And the word is 'soaring', rather than 'soring'. Soaring is 'flying'. Soring, if we used that word very much, might describe how you feel if you stop soaring, and crash)
Since this happens when the engine has been running for a while, I will assume it has something to do with the heat generated. Heat can cause engine loss of power and roughness: (1) If you begin to evaporate the gasoline in the fuel lines or carburetor. We call this 'vapor lock'. Fuel injected systems seldom if ever experience this since the high pressures tend to keep the fuel liquid. With a carburetor, the liquid pressure is less, and if the fuel boils in the line or carburetor, the engine will lose power or stop. The fuel can even boil out of the carburetor bowl, stopping the engine. If this is a problem, you may have to rebuild or adjust the carburetor , cool the fuel lines, or increase the delivery pressure.
(2) If you have an electrical part or system which malfunctions when hot. You have a very minimal electrical system here, so I would suspect any electrical failure would have to be in the ignition system....coil, capacitor, ignition control module, sensor...whatever you have installed on that engine.
(3) If you lose oil pressure, and the hydraulic valve lifters cease functioning as they should, you can experience a loss of power, clattering of the valves, etc.
(4) On some of the older engines, the ignition advance curve was controlled by vacuum from the manifold and by mechanical weights. The weights seldom (but occasionally) gave problems. The vacuum advance system required that the vacuum diaphragm was working properly and that you were connected to the correct source of manifold vacuum. Sometimes it was at the base of the carburetor, and sometimes there were more than one ports which were selected by a thermostatically controlled valve. Be sure you understand the ignition system you have and that it is working to specification under operating conditions.
These some of the things that have come into my mind. Maybe others will recognize the problem exactly and give a better answer.