1998 Dodge Neon had misfire code

When I went to get emmission check car failed had a code of misfire 303
had value cover gasket replaced because of oil leak in to plug tube. Then replaced plus and wires and then shop called back said stiill having misfire. They told me that coil needed to be replace showed that it was firing weak. Repalce this myself. Went back to get it rechecked. The tech did a precheck was ok but during the online check engine light come on and fail with a code 303 again. Came home unhooked battery and let it set for 30 mins. And then hooked it back up and have drove it for 100 miles and no check engine light come on but I done this before and got the reading code of 303. Is there something that I might be over looking. I have done vaccum checks and they were ok. Right now I am upset with the shop because I feel that they were doing trail and error which I can do at a lower cost. They said that they ran it on the computer before they started the work. Should that have not pinpointed the problem for them?