HELP! The 5.0 has an ignition miss!?! What is it??

HELP! I bought a '93 Cobra not overly long ago and decided to change the plugs along w/ the cap and rotor before heading off to a car show. After replacement the car seems to run fine for most of the day. Later that afternoon it begin to have problem in the upper RPM range. At 4,000 to 4,500 rpm then starts to cut out and sputter in all gears. The car has 70k miles and had the original plug wires. I took off the original rubber boot and changed out the plug wires thinking that would do the trick but no luck.

I removed some old split fire plugs and installed Bosh Platinum’s, along with a replacement cap and rotor from AutoZone. As mentioned it starts and seems to run fine (maybe with slightly less power.) Over 4,000 rpm it really begins to cut out. Haven’t checked the plugs but plan to shortly. Figured if it was a dead cylinder (er plug) then it would cut out across the rpm band? What’s going on!?! Anyone else had this problem?

Thanks!