Car wont run well consistantly. Very sluggish. What is wrong and what can I do to correct?

Was having problems with the clutch in cold weather on my 1998 3000GT. Put it in the garage for about 1 year. We seem to have fixed the clutch problems and changed the plugs and wires. Left the old gas, oil etc. In. Sometimes it starts right up and runs pretty good and other times it will barely start, takes multiple tries and when it does start it is very rough, misses like it is not getting a steady flow of gas and will barely go but if I continue to drive it/limp it along when it is acting like this it will finely come out of it and run fairly smooth. Problem is I never know when this is going to happen. It is totally unpredictable. We put lucus in the gas tank each time we fill it up in case it was the old gas that is causing it but I have driven it over 400 miles and it continues to act up. Do you know why and if so what can I do to fix it? Would the sending unit have anything to do with this?