For the past 3 days, my toilet almost overflows in the evening but no other time. I’m away at work for about 9 hours, but it flushes normally in the mornings and fine for the first few times when I get back home. At some point, I will flush, and it almost overflows but slowly returns to a normal water amount in the bowl. I can flush a couple more times, and it returns to normal. What could be causing this back-up just on occasion and not all the time?