Toilet was installed less then a year ago and has been working fine. All of a sudden, when I flush the bowl fills almost completely. It then drains slowly and almost completely. It takes maybe a minute or two to drain down to where there is almost no water left in the bowl. I tried plunging it a few times. I could hear water moving in the sink drain, but still get the same results when I flush.
It sounds like there may not be a problem with your toilet but a possible blockage or restriction in the pipe work. If there are any drain covers outside your house (generally close to your outside walls) lift these and check for blockages. If the drains outside appear clear pour a bucket of water down them to see if the water backs up, if so look for another drain cover in the direction of the flow and repeat the process. If the water flows freely then the blockage may be in the other direction back towards the toilet. In both cases drain rods can be tried to break-up the blockage. If it's serious/stubborn it may need jetting professionally with high power water. Good Luck!