It could also be the weather changing... getting hotter. My '47 PW has a running temp limit of about 85F with all stock style fuel system stock engine. Above that and the engine compartment including the lines that feed the fuel pump get hot enough after running for a while to vapor lock and she shuts down.

I fixed it with a booster electric pump on the rear frame rail. It operates on a toggle. When she starts stumbling, throw the switch and all is well. Also good for starts. I don't have to crank forever after she sits and I don't have to hose down the fuel lines and fuel pump with cool water for hot re-start.