Super C runs perfectly - then sputters - then runs perfectly

Just got a 1053 Super C. Tractor ran very poorly when purchased. Changed plugs, points, wires, an rebuilt the Zenith carb. It now starts immediately, idles smoothly, and makes good power.
One thing I cannot seem to correct is an occasional sputter. Usually the tractor warms up with no missing, then occasionally misses at any speed range. The new plugs indicate it is running slightly rich, although no black smoke can be seen.
Compression is 120 on all four cylinders.
I did not reset the float - could this be the problem?
Thanks.

my c will occasionally need the fuel line, sediment bowl and so forth cleaned; especially after sitting over winter.

float possibly to high or sticking, make sure there is no varnish gum in the bowl of the carb, this is classic issue with old tractor engines

What brand plugs?

Autolite 388 plugs.
I did not change the distributor cap and rotor, as there is no tag indicating whether it's a IH or Delco or Prestolite distributor. The cap and rotor look just fine.
Fuel delivery is good.

Ours always runs a tad rich, but runs so well I don't wanna mess it up, as mentioned, check the fuel bowl see what the sediment situation is, if you have an inline filter on it check to see if its getting any rust sediment in it, the look at the inside of your tank, see if you see any crusty rust or debris in there, we usually end up etching and coating the tanks on anything that sat for extended period of time. I also had a 240 once that drove me nuts, would break up and stop when headed down hill, a leaf of some sort had gotten in the tank, survived the gas and attached itself to the tank bottom by the pickup, fell forward over the pickup when facing down hill. Also the question of what brand of sparkplug is important too; certain brands just don't agree with certain tractors...our Super C seems to like Champion D21 plugs the best, but isn't shy about letting us know when she's read for some new ones (as I said she always runs just a tad rich) she doesn't like other brands much at all except when I'm pulling and using VP racing gas, this is assuming that since it's a Super you are running a distributer not a mag, if you do have a mag, you will want non-resistor plugs.

A Junk Condenser can give you fits. (A lot of the new ones are junk too).

Check the valve lash adjustments when you get a chance too.

Does pulling the choke out help?

(quoted from post at 17:07:41 07/26/14) Does pulling the choke out help?

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No, pulling out the choke doesn't make any difference (except doing what a choke should). I have yet to use the choke to start it. Turning off the fuel doesn't affect the sputter either (until it runs out of fuel).
Haven't checked the valve lash - which according to my shop manual is 0.014" hot.

Set the valves at.017" cold. Bring No1 piston to tdc on the compression stroke then adjust valves No 1-2-3-5 counting from the radiator. Rotate the engine one full turn and adjust valves 4-6-7-8. Hal

How much have you run it since you did the tune up? These old engines tend to be a little cranky with old gasoline and when not run. Sitting up tends to make them run rough . . . for a while. Hook a load to it and go to work. Every time it starts running rough stop and drain the carb, blow out the fuel line and clean the bowl under the tank. It usually takes about a tank of gasoline to get it going right. (This the way I had to do it every spring back in the day). BTW if you have a rubber hose instead of the steel fuel line that was stock that could be an issue. Rubber fuel lines seem to have loops and will drop below the carb. Air bubbles can get trapped in the loop and cause the engine to run rough even with a clear carb and good fuel.