Rondo Stech

I have a 2000 GMC Savana with a 5.3L 350 in great shape but sat for a few years. It was running great until the fuel pump went out. Ever since then it has been misfiring. I had a full tune up. Ignition (full) wires, plugs, rotor cap, everything and still misfired. I replace all the injectors and it still misfires. I replaced the computer as it was acting up, and still it misfires. It is now random misfire code for #2 & #4 (or) just #2. I have brought it to 2 different mechanics and they are dumbfounded. I found a better mechanic who fixed and checked over everything and still cannot figure out why it is misfiring on cylinder #2. I have paid out $1000.00 of dollars to get my van fixed and it still misfires. What am I missing? As the code says misfire on #2 or sometimes it says #2 & #4. At times when the light is off it runs great, good gas milage, but the light will just come on to service engine soon, and the misfire happens, then I get crappy gas milage. I am a mechanic myself, but only know older vehicles and a little of the new stuff, and still cannot figure out what is going on with my van. We did a smoke test and found a air leak to the gas tank, fixed it, and it still misfires. I am at a loss, please help.