Engine craps out under load and misfires sometimes

I hope you guys can help me, I have a 98 Mercury Mountaineer with 5.0 V8 . About 2 weeks ago, out of the clear blue, the wife called and said it was missing alitttle under a load. It seems to be at lower RPMs under load, you can feel it crapping out like it's a bad coil or wires. I checks the wires under hood after dark to try and find if it was arcing on valve covers, Nothing. I just changed the coils out and still, no change. I haven't had any engine light or anything, Nothing. I dont have a code scanner but didnt think I needed one yet because no light is on. Now Im thinking a Converter bad??? Dont know, Im taking shots in the dark at this point. I did notice that under very heavy load, I can bearly hear some back firing ( or Pop ) in the exhaust kinda like Ignition problems or timings off, but this engine doesnt have a distribitor. I hope someone out there has had the same problem and knows the fix. HELP! HELP!HELP!

Thanks,Larry