I have a 2003 Hyundai Tiburon, and the past 2-3 weeks when I first start the engine and either back up out of a parking space, or back up and pull forward, the car will not be under any power for a couple of seconds after I step on the gas, and then it will be just fine. It only happens when the car has been sitting for a while. My diagnosis (I know dick squat about automobile engines) is that most likely the fuel injectors are clogged. My question is do the fuel system additives by STP and Valvoline work, or should I get it done professionally by jiffy lube or some other mechanic?

is it an automatic? When you say no power, you can rev, but the car doesn't move?

If yes to all those then you are having torque converter drain back. The car has to re fill the torque converter before you can move. Common old car problem. Not seen it in a new one, but that sounds like what it is.

Quoted: is it an automatic? When you say no power, you can rev, but the car doesn't move? If yes to all those then you are having torque converter drain back. The car has to re fill the torque converter before you can move. Common old car problem. Not seen it in a new one, but that sounds like what it is.

It is an automatic, and when I said no power I mean that the car does not rev. It is a 2003, and the problem seems to worsen when I start it with the AC on. I'm going to try the fuel additive and see how that works.

still doesn't sound like a fuel issue. Most injector and pump issues get worse as you go faster, or rev higher. Not at a low idle when you start the car.
Now it could be a pressure problem as one person sugguested. That would mean a bad relay or something. When you first turn the key, the fuel pump should kick in and pressurize the fuel rail. Most times it is already. If your injectors leak down after a day or so, it could take longer, but that normally results in longer cranking time to start.
If that is the case, then perhapes injector cleaner might help.
If you can, get a pressure gauge on the shrader valve on the rail and see what it reads when you hit the key but not start the car.

The best fuel injector cleaning we've accomplished is with xylene. Run a mix of 1:3 gallons of xylene with the highest octane gas you can get. Just don't spill it on the paint. ;)

Oh and the car runs like a scalded cat on that stuff.