2000 GMC Safari

3 months ago I supposedly had a total tune up. I bought all the parts and the shop put them all on which saved me over 400.00 in parts. We picked up the van and barely made it a few blocks before it sputtered, jumped and acted like it wasn't getting enough gas and we litterally coasted back into the shop. Had to leave the van and replace the distributor cap, plugs, wires, rotor button. With in a week, the check engine light came on and the van felt as if wind would gust up against the side of my van. Of course I called, took it in to be diagnosed. Was told the 1st time it was the catalitic converter. Took it to a guy that has done converters for 25 years and he said it was not that but he would gladly sell me one if I liked! The next time I took it back to the shop the diagnosis was the gas cap. Then it would act up when it would be wet outside. It acted like it wasn't getting gas and would do alot of hesitating and sputtering. To make a long story short I took it in and told them to find out why after a 900.00 tune up (this also included the plugs wires etc) my check engine light came on and has stayed on for 3 months and my van is not running correctly. They called today to say that it is my wires, rotor button, and distributor cap again! Could it be that these were put on wrong to begin with and that is why is has run badly from the start? Isn't there something called a "firing pattern" that has to do with the wires that if not put back correctly can cause the vehicle to run badly? What is the firing pattern for a 2000 GMC Safari? I am not very knowledgeable about cars as you can tell and I am very frustrated with this shop. This time around they want 489.00. Thanks for your help.