Fuel problem?

I have a 97 plymouth neon with about 88,000 miles. My check engine light came on a couple months ago and it read multi-cylinder misfire. I replaced the spark plugs and wires, the fuel pump was replaced about 5 months ago along with the ignition coil. I also replaced the fuel filter. I have a hole in my filler neck about an inch wide, and I covered it with a clamp for the time being. My car runs ok for about a week, then it starts acting up. It jerks and shakes and if I keep it running it will stall out and wont start. Then I wait a few days and it starts up and runs ok again. I've been told the fuel injectors may need to be replaced, and that it's quite costly. Ive tried putting gas treatment in, as well as fuel injector cleaner. Do you have any other guesses on what could be the problem?