Do these periods of no smoke coincide with the tractor doing some heavier work? Is the engine running hot enough?
Presumably only blue smoke? Oil going down valve guides? Is the fuel in the oil from ticking over? Might be an injector spurting rather than spraying.
If it starts well, there is likely not a lot wrong with the rings and bores - unless only on one or two pots. Removing manifold and running the engine might give some indication.