This reminds me of my first car, a '74 Datsun 260Z. In hot weather, the carbs suffered from a condition referred to as "vapor lock". It would sputter along in hot weather, especially when the engine was warmed up. In cold and especially cold damp weather, it charged like a bull. I loved that car.
But I digress. Sounds like a fuel delivery problem of some sort. I would try a couple of tanks with Techron, then go for a nice Italian tune-up.
Somebody jump in here that knows what they are talking about.