What was the engine temperature like? Something similar happened to me. Started and ran fine until a good run heated it to the point the gas vaporized and perhaps the cylinders/valves got a little sluggish. The rad was full and the temp gauge inaccurate leading me to believe it wasn't heat. The problem was a very old rad that, despite not leaking, had collapsed on its insides and was not cooling by way of the fins as it should. Let is sit and cool for 20 minutes and it started like new. Replaced the rad and the problem went away.
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