Fuel tank

I have a 95 Buick Lesabre it has 89,000 mile v-6 that will be getting a new fuel tank next week, I have a pretty good leak. This past week my "check engine" light stays on and when I start the car it idles very rough and then smooths out and runs great, terrific pickup and idles normal at lights. No missing whatsoever. Some days I start it and it starts up great. Question. Is the problem caused by a leaking fuel tank, is it loosing pressure from that or could it be something else. I am newly divorced and really do not want to go to my mechnic knowing nothing. Thanks.