For a while when I flushed the toilet, it would flush fine sometimes but I usually had to use the plunger. Now when I flush it, the water level seems ok for a few minutes then slowly drains down so that it only remains in the small hole at the base of the bowl. It makes a gurgling sound off and on few 5 or 10 minutes. I have used one of those products that you pour into the bowl and let set for a while then flush later. It worked for a few months but now it is doing it again. What can I do to fix the problem?