Spark Plug # 4 keeps going out
A few months ago I noticed the transmission indicator seemed off... it kept getting worse so now if it looks like I'm in OD it's actually in drive - if it looks like it's in drive, then it's actually in neutral. Don't think that's related to the rest of the issues, but just in case...
About a year ago the check engine light kept coming on, so I took it in and they said my #4 spark plug had gone bad and replaced it. Since then the check engine light has come on several times and each time they've said it was an oxygen sensor and have replaced three so far... At that time the truck was only driven about 2 miles per day (35 mph) so I think that might have been part of the issue.
Now its' been misfiring badly... its very rough when it starts up and it smoothes out a little bit, but generally at around 50 mph it is extremely rough. Now the truck is driven about 6 miles per day at speeds of 35 to 50 mph.
The check engine light has been on and it said code PF304 which I understand is spark plus # 4. But it was changed when we had this problem last year and I know it's rated for 90,000 miles - and it's probably only gone 5,000, if that.
Any suggestions on what we can check or what might cause that spark plug to go bad repeatedly? Could it be related to the gear indicator not being correct? And could that be a larger problem in itself? We just bought a house so I'm really hoping we can fix this ourselves without huge mechanic bills!