I have a 2004 ranger with the 3.0 v6. This engine had a burnt valve at about 48000 miles. I had it fixed and now at about 98000 miles Iseem to have another burnt valve. I dont remember the code but it was random misfire cylinder 3. Compression is around 20 psi lower on this cylinder. The plugs look new. The mechanic said they looked like fuel had never touched it. Can you tell me what is causing this? Is there a known problem with this motor?

Thanks for anything that may help.