Chevrolet Repair: missing at idle and backfire, chevy c1500, rocker arms

Question
i have a 1992 chevy c1500 with a 305. It misses at idle and when you rev it up 2 or 3 times it starts to backfire. it has new plugs, wires, cap, coolant sensor, new map sensor, intake gasket, coil, but i can't seem to figure it out. i was told maybe the timing chain but its good.. any ideas?

Answer
Hi David,
with the information you gave me I am assuming it's backfiring through the throttle body not exhaust.  If it is backfiring up thru the intake system my hunch is that the cam has got some flat lobes on it and it's not opening the valves enough causing it to pop back thru the intake.  The best way to tell would be to take the valve covers off and watch the rise of the rocker arms.  And I think you'll find that one or two or three are not moving as much as they should or else a rocker arm, or rocker arms, have split.  
Hope this helps.
Tom