Car runs worse after tune up than before -- mis-firing

My dodge is a 2002 caravan with approx 209,000 miles on it. It is a 3.3 liter engine. I have never had any real trouble with it before this. I bought it with 84,000 miles on it and ig good condition. I have the trans power flushed every year and oil changes every 3-4000 miles. I have a small oil pan leak but other than that the van has ran well. It was time for a tune up and I purchased all the items for the tune up. Plugs, wires, coil pack, oil, oil filter, air filter, serp belt, etc. However, Checker sold me the wrong plugs for the van and since placing those plugs I was getting a mis-fire in cyliders 2 and 5. We replaced the plugs with the ones that were supposed to be in the van, and that did not fix the problem. I took it to another auto parts place that does mechanics as well and they were able to "fix" the problem and for the last week it has ran GREAT! However, the van's check engine light came back on last night and started hesitating and sputtering like it did before I got it in to the mechanic, They ran a diag on it but found nothing within the engine that would be causing the trouble. Just the fact that the "bad" plugs did something to a wire on the coil pack and they fixed that when it was there. PLEASE help! I am not sure where to turn now and I do not want to get taken advantage of because I am female.