Also, one last, and REALLY important item.
Take your car to a local mechanic, and have them remove the exhaust header/catalytic convertor, and see if the convertor material is damaged. If it is damaged, you will need to replace the header with another one.
If the material is damaged, it breaks apart, gets sucked into the engine, and causes damage to the cylinder walls of the engine, causing very high oil consumption issues, and eventual engine performance issues.
If you are having to replace a lot of oil in less than a couple thousand miles, this damage might have already occurred, and could be responsible for the idle issues.
This was a common issue on '02, '03, and some '04 Nissan Altimas with the 2.5 4 cylinder engine.
BC.