The jumper trick will only display codes that have set CELs in the past. If you don't have any CELs, your computer is probably clear of any stored fault codes. Your computer wouldn't tell a mechanic anything if you paid for a diagnostic check, so save your money on that. As far as lacking power for the first 20 seconds, I guess I'd say let it warm up for 20 seconds. (It usually takes at least that long for the idle to drop to normal when it's cold.) If it's sputtering or running rough until it warms up, it could be a number of things. Is this an automatic or stick? How many miles? How old are fuel/air filters and plugs/wires? If you can provide a bit more detail, people will have a better idea on what may be wrong. My first guesses would be air leaks, fuel filter, air filter, plugs, and then wires. Possibly dirty injectors which may be helped by a bottle or 2 of injector cleaner over the next few fill-ups. Hope this helps.
