1998 Ford Focus hesitates and high revs when idling

Could be dirty throttle body. If so a dose of throttle body cleaner may help. It may require removing to clean or replacing in worst case.

Likewise with injectors.

Also look for air leak on inlet, any split or detached breather hoses etc.

Asauming plugs and air/fuel filters are in good condtion.

Also check plug leads , could be breaking down intermittently under load .

Idle control valve is a possiblity. If its misfiring- check plugs and leads but first check the coil pack as these commonly fail on these cars. Other suggestion cheap and easy try cleaning throttle body behind the MAF sensor or try using multivalve stp cleaner-yellow bottle (goes in the fuel tank). I've used this in my focuses (I've had 3 altogether)to good effect following the instructions stated on the bottle. Good luck.