When we flush our toilets (not just one) the water does not flush all the way down. It drains most of the contents, but it does not totally drain and you dont get the girgle at the end. Sometimes, holding the handle down the entire time the water drains works. However, not always. Also, the problem is not consistent. When we bought our house 8 months ago we noticed it in the basement and assumed it was a flapper problem. Then as time progressed it has intermittently happened to all our toilets. The house (and toiletes, I assume) are about 20 years old. Any idea what could be causing it?