Ford Focus, 1999, 1.8 zetec - idling lower then usual - mstar

Hi people,

my focus after years of careful maintenance has a slight issue. Its a 99 Focus, hatch 1.8 zetec, and i noticed that it SEEMS to be idling lower then usual.

As i dnt drive it much (i have my mondeo ST for normal drive), it usually idles around the 1-mark on the rev meter. but yesterday i noticed its below that by a notch (and i stalled once).

ANybody tell me if this is a big issue or not? is it a cheap part to fit?