I have several toilets in my complex that are double flushing. About 45 seconds after the flush is complete the remaining water in the bowl drains out like a second flush without water coming from the tank. I have pulled the toilet and snaked the drain with 75 feet of 3/8 cable and have snaked all of the vents from the roof. The one commonality between all of these toilets is that they all sit on an old style angled flange on the bottom floor. Anybody have any suggestions??