Ford Repair: Fouled spark plugs - again and again and again, spark plug wires, exhaust gas recirculation

Question
I have a 1998 Ford Contour, 6 cylinder.  In October I had a tune up, including new spark plugs.  Shortly thereafter the car started "missing" when acclerating.  I took it back and the spark plug wires were replaced.  Car ran fine for a couple of weeks, then it started missing again.  I took it back - the plugs were replaced again - and again the car ran fine for a few days.  Then, it started missing.   I took it to a different garage - they replaced 1 cracked spark  plug and ignition coil.  Once again, ok for a few days but it started again.  This time,  2nd garage could not find anything wrong but they replaced 4 plugs ( I guess for good measure).  (there were no charges for replaced plugs/wires as they were under warranty).  I continued to have problems but without identified source or solution. I finally took it to the Ford dealership where I bought the car.   They replaced a valve which was not "closing" properly.  I was back the next day - they had done nothing to fix what I brought the car in for - missing.  They evenutally diagnosed problem as 2 bad spark plug wires - I took car back to orginal garage and they replaced wires.  I thought the problem was solved - for the next 2 weeks the car drove fine.  But then, once again, a bad spark plug -- and again.  No one seems to be able to figure out what is causing the spark plugs to "foul out".  The same plug has been bad at least 2, maybe 3 xs, but the 2 wires that were bad were not connected to that plug.  They are not finding oil on plugs; I asked if there could be a clogged valve (because I read that this could cause plugs to go bad) but this apparently didn't seem to be a consideration.  There is no engine light unless the car misfires (when the engine light will flash; 2 x this led to engine light remaining lit and resulted in new ignition coil and valve replacement).  Some one suggested I have the Exhaust Gas Recirculation checked - but I can find no information that this would lead to spark plugs going bad.  I can not believe that it is good for my car to have spark plugs/wires "fouling out" every few days.  Do you have any suggestions thoughts - PLEASE!

Answer
Hello and thanks for the question...I'll try to help.
 There are several things that can cause a plug to foul. What I suggest you do is check to make very sure that the plugs are not getting fouled with oil or antifreeze. Next I would have the injectors leak checked. And also have you EGR system checked out to eliminate that the valve it's self isn't leaking exhaust gas in to the cylinder when it's not needed/required.
One last thing, it may be silly, but change your oil! Oil that is fuel soaked, slips past the rings, more so then new/fresh oil, and can cause problems.
Hope this helps,
Erik