Pushing it feels like someone keeps pressing a brake

Ok, I'm sure this may sound odd but I'll do my best to describe the problem. I have a 30612 which I think is very similar to the 34612. When I push it on the carpet or on the wood floor it feels like a brake is being applied on and off repeatedly. In other words, it's hard to push and then easy to push. In about a four to five foot push it will be easy-hard-easy-hard-easy-hard. Sounds goofy, right? This will repeat no mater what conidtion. It doesn't matter if the vac is on or off, or if the lower housing is released or not. I think it's either the inside diameter of the wheels have worn to a point that they are sloppy and twist out of alignment or the wheel shaft is not being held properly by the plastic dust housing (due to wear) and it allows the wheels to rub on it.

Anyone seen this before?