2004 Jeep Cherokee Spark Plugs keep going bad

I have a 2004 jeep Cherokee with about 85k miles on it. I did a tune up on it a few months back, replaced the normal spark plugs pvc valve, air filter, fuel filter and so on. Everything was great for about 2k miles (about 2 months). I was on the free way driving from San Diego to Tucson az and was about 4 hours into the trip, when the check engine light came on, the car started running rough and maintaining 65mph was no longer an option. The light started flashing so i pulled over. I waited a few mins and started driving again. Everything was good until i hit 40mph. Once above 40 the car would start shaking and would run rough, like it was miss-firing. I stopped and had the jeep towed to a shop, where they looked at it, pulled the codes and said it had a random multiple miss-fire. They pulled the plugs and said a couple of them were fowled. They went ahead and replaced all the plugs again. Everything has been fine until today ( a month after they replaced the plugs). The car is having the same symptoms, (running very rough, hard to maintain above 40mph and the check engine light is flashing). I had the work done in Tucson so i cant drive it back there since i live in San Diego. So i am wondering what is causing the spark plugs to keep going bad, (im assuming its the same problem, since the symptoms are the exact same). I am unable to pull the plugs and look at them right now wsince im not with the car. Thanks for your help