With intermittent issues like what you have can best be solved by driving it until it gets worse.
I have had an issue with a '53 235 with a loud hydraulic lifter until the engine reaches operating temperature. Finally it is now clacking even after fully warmed up. I can now pull the valve cover and find out exactly which lifter is making the noise and replace it.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission