Misfire & Backfiring?

I have a 97 Range Rover with a 4.0L engine that starts & idles smoothly, but whenever the rpm is increased from idle towards 2000 rpm the vehicle would start sputtering & backfiring, i notice when this happens there`s no spark going to plugs 2,3,8 & 5, if rpm is decreased these plugs would start firing again, i already swap my coil packs with my buddies his works & mine works in his, anybody out there has any other ideas as to what could be causing this problem.

Mike
February 2011