Any ideas?

I’ve got a 99 Ford Escort wagon that’s occasionally running on 3 cylinders. It’s idling fine, under load when the cylinder cuts out, it is usually preceded by a loud pop. So far I have replaced the coil pack, plugs, gas filter, and plug wires which hasn’t helped. My mechanic checked the new wires yesterday, and pulled out the injector wires while it was running and all the injectors are working - maybe one is weak? I’m stumped and don’t know where to look next - the ends of the plugs look fine. Any ideas???