All Of A Sudden Misfiring, Rough Idle

[getyayayaout]

Hi guys. Wouldn't mind getting some advice before i go throwing parts at it.

Went for a cruise with some friends, wasn't pushing hard at all but was driving for about three hours. On the way home I pulled over to use my phone and noticed a really rough idle all of a sudden, was jumping between 500rpm and 1000rpm. The headlights were flickering etc. Take off again and anything below 2000rpm is missing bad, and got a crazy backfire aswell which has come from nowhere. After 2000rpm however, there doesn't seem to be a problem, everything sounds normal and it still goes ok. So I haven't had a chance to pull it apart yet with work and exams but I really dont know what it could be. My mate replaced his coils and plugs in his 33 but his miss was in the top end over 4000rpm. ????