SLT1554 eating drive belts

I have a 2005 SLT1554. I bought it used in spring 2010. This is my 6th mowing season and I'm about to install my 4th drive belt. Though this is the first rider I've owned, I have a feeling this isn't normal. My dad and father-in-law have owned their JD's for about 10 years now and have never replaced their drive belts. There are two problems I've noticed that might cause the problem:

1. My deck has a noticeable vibration. It goes from loud to quiet and back to loud in about a 3-4 second cycle. I know I've got an imbalance in there somewhere, but I don't know where or how to find it. This problem has existed since I bought the machine, but I'm not sure if this could shred a drive belt.

2. I checked all the pulleys associated with the drive belt. All are spinning freely, but I noticed the double idler pivot bracket is not only moving side-to-side, as it should, but seems loose because it moves up and down also. I'm not sure if it's supposed to do that.

I've uploaded a video to YouTube that shows what I think might me the problem. Would appreciate any advice before I reassemble everything and shred another belt.

https://youtu.be/9R-zvdOfnvI