RPM slightly unstable

Fordies...
Few weeks ago, I changed my Focus air filter with Pipercross air
filter, especially designed for Focus 1.8.

Performace wise, very2 satisfy. 1st & 2nd gear can really fell the
different when I use the original air filter. At 5th gear.. the car
still "kick" (even though I've already > 100 km/h) when I
accelerate & can easily over-take with 5th gear.

The only problem (I think it is not really a problem?).... I notice
the RPM is slightly unstable when I am on free gear. My concern
is...
1) will it cause problem in the long term?
2) I understood that for Escape, Zetec engine will "self" adapted
to driver's driving style... is it the same case for Focus?

Fordies... your comment is very much appreciated.

Finally I got it right.

Initially I wanted to let the electronic system to re-adapt itself. It did the job but not to
100%, even after > 1 month.

Last week I checked with Ford mechanic if they can re-tune the engine to fully stabilize
the RPM. But according to him, Zetec engine is self-adapted & they can't tune it!!!!

Not satisfy with that explanation, I finally disconnect the battery & hope when I
re-connect the battery, the engine & electronic system will re-tune with the Pippercross
air filter. IT WORKED!!!!, within a day, my RPM is now back to normal (stable) & to my
suprise, the engine is now giving me even smoother ride!!!!

For Focus owner, if you make any changes, it good to erase the current set-up by
disconnecting the battery, rather than leaving the electronic system to re-adapt to the
changes. I think, this is valid for Escape also, b'coz Focus & Escape are using similar
engine family.