Rough Idle in El Salvador

First thing to check would be plugs and wires, how long sinse the last tune-up? You may just need the throttle body cleaned, if it has never been done before.

I changed the plugs, air and fuel filters and it did not seem to have any affect. If it we back in the 70's I would suspect a vacuum problem. Does this seem likely? When accelerating it runs fine, no hesitation, etc.

A vacuum leak may be a possibility. On fuel injected cars a small vacuum ussualy will cause an increase in idle speed as the computer is able to compensate to a small degree. A large leak would likely cause a hesitation and may turn on the check engine light. You can check for a vacuum leak by spraying carb cleaner at the intake gaskets and vacuum lines. You will notice a change in engine rpm's if you find the leak. You may need to hook it up to a decent scan tool that will let you read the sensor values while the vehicle is running to determine what is wrong.

Seems to be a universal truth.