Erratic Loss of 200rpm On Climb Out

Hello, my '69 has been losing around 200rpm and running rough after I get to 500' during climb out. It did it two flight ago, ran perfect last flight, then did it agin this morning. The header fuel cap vent is fully open and air filter is good. What could be causing this issue? I live in Florida and this morning it was 81F and 90% humidity. Could carb ice be a possibility? Any input will be greatly appreciated as I will not be flying the airplane again until the issue is diagnosed and fixed. Thank you so much.