Toilet: flushes slow when not flushed for a while

I have a strange toilet mystery here: my toilet flushes great if it has just been flushed. However, the flush gets progressively worse as it sits without being flushed. Let it sit for 20 mins, the water in the bowl only makes it to an inch over the top of the drain hole. Let it sit overnight, and the water in the bowl barely goes down halfway. The next flush (and any flushes in close succession) will be perfect.
I first suspected some kind of leak, but can't find any. The water levels in the bowl and the tank do not go down at all throughout these periods.
Any other ideas? I'm stumped.
Thanks, Joe
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Do you have any kind of 'disinfectant' product in the tank or commode?

just
without
flushes
in
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No, none whatsoever.

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Have you taken the Toiletology 101 course yet? http://www.toiletology.com/index.shtml
But you do get a passing grade in the Bluegoo portion.

the
levels
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Joe wrote:

If you don't know the disease, treat the symptom.
Have you plunged the bowl? Snaked the line? Poured really evil, fuming, de-clog down the drain?
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