During a bathroom remodel, I recently installed a new toilet. The old toilet worked fine, and this one did at first. Now about a week after it has been in, a strange problem developed. The toilet flushes as normal, then a few seconds after flushing the water level in the bowl begins to drop until it reaches the drain hole. Then the toilet seems to suck air for some time. I am looking for suggestions. I suspect it may be inadequate venting, but why the sudden start.