I had that exact problem. The belt kept slipping off to the right side and kept chewing the belt up (replaced every time). The first time it happened, I managed to save the heli. Second time, it quickly lost power, I had no time to bring it down safely, it locked up and fell to the ground.

Old Crashman is correct. There are two methods that "should" fix the problem.

Some believe it might not be a good idea to take the pulley out since align must have had some reason for having it in the design. It might fix the problem and not hurt anything, however... even safer in my opinion, what I did was got an idler pulley with raised edges as mentioned.

What I got is unfortunately discontinued
http://www.eboyztoyz.net/index.php?op=showitem&sku=6701

I believe I read when I was researching the same problem that this also fits and works perfect
http://www.eboyztoyz.net/index.php?op=showitem&sku=8844

No problems since changing it.