What is wrong with my car?

When was the last time the fuel and air filter were changed?Well start with those two if they have not been done in a long time.Lack of fuel at idle from a dirty fuel filter or air filter can cause the car to run like crap.

check vacuum lines. when you open the hood, you will notice small little hoses running throughout, a disconnected hose, or one that is leaking vacuum will cause problems such as that.

google search in images "01 altima vacuum diagram" it should give you a diagram that shows where all the hoses are and where they go. If you have a check engine light on, have that investigated as well, that will lead you to a confirmed problem. If you want to know what the check engine light means, take it to your local autozone, they usually have a scan tool you can use for free (they will walk you through it) and what causes that check engine light. You can now proceed to fix it yourself should it be an easy enough fix.

It sounds like your valves may need some re-adjusting. It may not be getting enough air or fuel after the engine warms up (your on-board electronics system manages this).

If you're unsure, go to a mechanic, he can hook up a diagnostics tool and have a look at the system settings.

since it runs good on cold start circuit, that *typically* points to a vacuum leak.

have some one you trust do a cursory visual in the engine bay for any vacuum leaks. hoses, fittings and such.

otherwise, gonna have to take it to either a reputable ASE shop or the dealer for diagnosis.

on the surface, it doesn't sound serious. maybe an induction/emissions systems, and physical sensor cleaning.

complex issue look into from search engines like google that might help

Best thing for you to do is, take it to a shop

Have it test driven and fully diagnosed

They're the professionals

Only they will know