I have an AS toilet from the 80's, Tank 4083. I have replaced 3 flappers in the last few weeks. Evidently, I am not getting a good seal because about every 5-8 minutes, the toilet runs like the last several seconds of a flush cycle. But every time I replace a flapper, I run my finger around that seat to make sure there are no obstructions, etc. I installed a flapper specifically for AS toilets today and still the same issue. I have three of these units and this one is the most troublesome. What do I do next?