Hyundai Repair: 2001 elantra Rough idle, throttle position sensor, air temperatures

Question
I have a 2001 Elantra that has recently had the spark plugs and wire replaced.  I have vibrations when the car starts, and they normally stop after 5-10 minutes of driving.  The car will then run smothly for about 30-45 minutes and the vibration will return.  When i am at a light the car will be at 400 rpm and suddenly drop as if the car has lost all power, the loss will take place for less than a second and go back to normal.   - The second problem is after sitting for 4+ hours, When i start the car the rpm is at 1500  and will take 2-5 minutes to drop to 900 (when i feel it is safe to drive) It will then take 5-10 minutes of driving to get the idle down to below 400.  At this point i have about 20 minutes before the first problem begins to kick in.  Please advise  

Answer
If your engine is idling at 400 RPM or lower, that's far too low.  Normal idle is at about 700 RPM when at operating temperature.

Also, at colder temperatures, it's not uncommon for a vehicle whose engine is cold to idle at 1500 or so.  The higher idle is necessary to keep the engine running at colder air temperatures and assists in warming up the engine quicker.

It's also possible that you have a problem with your throttle position sensor.  This was a common problem with '01 and '02 Accents and Elantras.  A throttle position sensor reading improperly will cause abnormal idle conditions and may set a check engine lamp.