Why would my chain jump?

This is an '08 and the rear sprocket and chain are pretty new. I put a new front sprocket on (15t), but it wouldn't fit with the chain slider, so I modified the slider. There's not much room around the 15t, so I was a little paranoid. I've never tried this with my 'berg, but I ran it in first gear on the stand, just to make sure everything looked ok. But, the chain is jumping quite a bit. So, I put the old sprocket back on (14t- still in excellent condition, shows no wear). I get the same jumping.
When I give it a little throttle, it seems to jump a bit less, but still jumps a lot. (I didn't want to give too much gas, because I kept picturing my video on one of those "funniest" or "stupidest" shows when something goes awry.)

Here's a video:

https://picasaweb.google.com/1179750124 ... 3261146322

(I don't know why I can't embed anything on this site.)

I posted the same thing on adv, and they basically said the same thing. It's nice to get a little confirmation when you are unsure. Since I'm in NYC, I don't have a back yard where I can go for a quick test run, so I like to try and get things sorted before I head out.
thanks