You might have a bent cog tooth. Carefully inspect it for bent or damaged teeth. Keep in mind, some of the teeth are cut at a bit of an angle to aide in chain pick-up when changing gears. Just look for one or two teeth that are out of line from the rest.
Also, be sure you have the correct chain on the bike, and make sure there are no stiff links. I suggest investing $10 in a chain checker. You can use a ruler to check your chain, but it's a bit trickier. Basically, each link pin will fall on a half inch mark. If it's off by 1/16th of an inch over a couple feet, replace the chain. If you don't care for your chain properly (keep it lubed up well, and don't use degreaser on it), you can actually wear it out in a month and a half. When it wears out, it wears out your chainrings and cogs, and it becomes a lot more expensive to fix.
I'm still unclear on your symptom. So, the chain sucks at the top of the cogset when backpedaling? That can also be caused by a freehub starting to get gummed up. As in, when the wheel spins forwards when rolling down the trail, and your cranks are stopped (coasting) a freehub that isn't spinning freely enough will want to turn the cogset forward, making the top (the driver/tensioned) part of the chain go slack. If you take your rear wheel out, (therefore, isolating the chain from the cogset) you should be able to spin your cogset around pretty freely. While you got it out, make sure there is no bearing slop between the cogs and the rest of the hub.
On reading your posts again, it seems as if the chain is riding between the cogs, is that right? Kinda feels like you're in neutral? I say first check to be sure your derailleur didn't get smacked in sideways. It should be perfectly vertical compared to your chainrings. You can sight down the length of the bike from the derailleur to the big ring. They should be perfectly parallel to each other. Next, check the derailleur adjustment to be sure the chain jumps up and drops down easily with each shifter click.
Check the Park Tool website, too. There are lots of how-to's about diagnosing and fixing issues with your bike, along with maintenance how-to's.