RM-Z450 backfire on shutoff and other issues

Whenever I hit the kill switch on my bike, it backfires once it stops. It is definitely coming out of the exhaust, not the intake. So I figured my idle was too high. I turned it down and the backfire on shutoff seemed to go away. After the last ride I noticed there was a little bit of oil around the valve cover gasket. I cleaned it off and started the bike to see if it would come back. The bike would occasionally idle a little higher than it should and I would have to rev it to kick it back down. Eventually when I shut it off it backfired again. Any suggestions?

Another thing I thought of...

When starting it, it will usually catch and run for a few seconds then die. It will do that a few times until it finally catches usually.

A while I guess... I have out about 10 hours on the bike since I got it, and the shop I bought it from said the engine was just rebuilt including new valves, so maybe they need adjusted?