2001 chevy 1500 truck flashing P0300 code

Question

My truck has a 4.8 litre in it and is flashing a P0300 code.
I have read up on the code and replaced my spark plugs and wires. i have also cleaned the throttle body at the suggestion of my mechanic. The code only flashes when i drive the truc own the highway at 90 km/h in the 3rd setting not the drive setting. as i said ive put new plugs in it and it still comes up every now and then, id like to get the code gone and out of my hair but am working on a shoe string budget being a teenager, so changing all of the coil packs isnt a viable option right now. can anyone help me further diagnose possibly which cylinder the problem is so i can change the one pack not all 8? thanks guys