Wierd. I've never seen that happen with a properly gapped new plug. If it backfires, it has to be electrical...
I'd try running some fine grit sandpaper (220 works) through the points, spray them with carb cleaner, then run a piece of paper between then until it comes out clean. Sometimes a tiny amount of oil can get on points, and although they look clean, the plug doesn't fire every time. Once the bike runs a little, the points are opening and closing so fast the oil can't ooze back.
Other than that, check the wiring over to see if anything is burnt/worn/internally broken