2002 focus doesn't idle at stop but then acts okay sometimes

Restricted exhaust . Catalytic converter or muffler may be restricted .

Dirty air filter, dirty throttle body...try 2 bottles of gas line fuel injector cleaner...has your car been parked for awhile? Heet gas additive will work on any low gas tank sludge that you may have. Hope this helps.

Clean the throttle body and check or replace the idle control valve. Also check or test the throttle position sensor.

Rough idle / stall at idle Check for vacume leaks & split intake tube or loose clamps on intake. Noise put spot of grease on drive belts if noise goes you may need new belts or a tighten up.

could be coil packs, or maybe a camshaft position sensor.

If you get rough idling or the tachometer needle does not stay in one place when idling then you may have a dirty or faulty idle air control valve. This is a common cause of the problem you are describing. Other issues could be dirty MAF sensor or loose wires or vacuum hose.

you have a vacuum problem, I would have the intake gasket checked or you can spray carb/choke cleaner around the intake where it meets the heads and if the rpm of the motor reacts then you have a leak, my explorer was the same way, idle raised and lowered,idled rough, stalled, and my vents acting up. new intake gaskets a month ago and wham runs great. My yukon i did 2 days ago as it had the same actions and yep it runs great now. I'm sure its your intake.

could be your clutch is not fully disengauging or on its last leggs check if you can ajust the freeplay at the clutch cylinder thats if your car is a manual

U DON'T HAVE A CARBURETOR SO FORGET THAT, THE FUEL REGULATOR WOULD NOT CAUSE THIS PROBLEM. I AM A GM FACTORY TECH AND KNOW THIS WITH CERTAINTY. U COULD HAVE SLUDGE IN THE IDLE SPEED CONTROL AIRBYPASS VALVE.

Here is the most common cause of surges and stalls and low idle rough idle, it is the idle speed control air-bypass valve and throttle valve (IAC for short), they get full of gunk over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.