Sounds to me like you might be slightly out of time.
Easy couple of diagnosis things you can do though:
When its missing at idle, put on a good insulated glove, and pull the spark plug cables one at a time. If a cylinder is sparking properly, you should hear and see it arching as you pull the wire away... At this time the motor should slow way down and lag as if its about to die, as its just lost 1 cylinder. Quickly plug it back in after it does this. If you pull a plug wire and it doesn't change the way the motor idles, then you know that cylinder is what's causing the issue.. Check spark, fuel and compression to determine why that cyl is down.
It might help you to check your ignition timing. If you're getting a miss PLUS a backfire, it sounds like you might be slightly off. Put a timing light to it and see if you need to move your dizzy around.
Looks like you've got a timing chain motor? So chances are you didn't slip a tooth there unless you've done some work lately.
Just some place to start, if ya get more more specific info, I'll follow this thread and see if there's anything else I can think of.