Your overflow tube does not have any cracks does it? If you know 100% for sure that your flapper is not leaking and the water is not turning on in between then the only thing I could think that would cause it is a crack in the overflow tube that is just enough to do what you say is happening or it is turning on and you are not noticing it.
jexmex t1_j2es5bk wrote
Reply to Toilet trickling water from jets, but not a leak! HELP by Aqueousfun
Your overflow tube does not have any cracks does it? If you know 100% for sure that your flapper is not leaking and the water is not turning on in between then the only thing I could think that would cause it is a crack in the overflow tube that is just enough to do what you say is happening or it is turning on and you are not noticing it.