305 engine issues

[trac209]

Im absolutely stumped here,I have a 305 in my winter driver and it's giving me fits to no end and I'm out of ideas. In short all of a sudden without any changes made by myself it has been bogging and stumbling and rough idle,and it seems intermittent. I first looked at carb all seemed fine fuel filter,choke etc, then took a look at hei cap and the carbon button was broken off and just fell out when I lifted the cap. So I installed a different cap rotor and After a short ride it cleared up and was running super smooth and without a hiccup.

That was yesterday,went ice fishing today about an hours drive away and it ran like new the whole way there. Then on way home it started doing it all over again. Bucking and surging on takeoff and only ran decent if I could stay on the gas and even then it was bad.

I'm completely lost at what this could be due to its intermittent nature,the only thing I could think of ice in the fuel line but once you can step on it hard it will accelerate hard, once you try to cruise its crappy. Any thoughts or ideas would be super helpful,thank you