2001 Ford Focus eractic idling

Is your 'check engine' light on?
I would check for a DTC or pending DTC; you may have a misfire going on.
If you do check the following:

- vacuum leak (vacuum test)
- EGR valve
- check your ignition system
- check for proper compression
- check for good injection

Also, some idle problem are related to a faulty or dirty IAC valve (clean it or replace it).
Get you some throttle cleaner, and with the engine on (rev at 1500 rpm), air duct off, open the throttle and spray the IAC real good for a few second. Turn engine off and let it soak for a few minutes.
Turn engine back on and spray the valve again (engine rev at 1500 rpm). Turn engine off, let soak and put air duct back on.
Turn engine on and hold at 2000 rpm until no white smoke comes out of tail pipe.
If it doesn't fix your problem you either have a bad IAC valve (replace it) or this is not the problem that's causing the rough idle condition (suspect a misfire)