If I'm thinking of the correct odometer, way back when I had both odometers begin to not function and it was the little white intermediate gear between the two odometer end gears having slid out on its shaft a bit. As the little gear moves out and is only barely catching the teeth, I think it was the trip gauge that started to misbehave first. Simply pushing the little gear firmly back on the shaft was the fix and I never had the problem again. If they both misbehave together then your suspicions are probably correct, the large main gear. Normally about 1/8" of shaft is sticking out beyond the little white gear, so before you button everything back up, double check to make sure it's pushed on all the way. If it was a missing/damaged tooth on the small cog gear inside the large gear then I would expect it wouldn't be an intermittent problem. Applying glue with a toothpick or whatever rather than straight from the tube will help prevent over-runs.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now