At times my car is good but at times it starts to behave bad. Ihave changed the fuel pump and filter. Cleaned the injectors. I put a new o2 sensor.Its rpm is at 500 and then 1000 and back or lower to the point that it shuts off. What else could be giving me that problem?

. Ensure spark plugs (Motorcraft) and spark plug wires are relatively new.
. Check/replace Ignition Control Module and coil pack.
. Check for codes.

. Check for vacuum leaks.
. Check/clean Idle Air Control Valve (IACV) and hoses--I use brake cleaner.
. Clean throttle body with Throttle Body Cleaner approved for teflon coated throttle valves; e.g. Valvoline Throttle Body Cleaner or 3M's Throttle Body Cleaner.

If the problem persists, read the link below for ideas, even though it's primarily written for Hondas:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-46563-7.html#post842969