there's a few things I think of off hand that will cause it

Idle Screw not properly adjusted
Vacuum leak
Stuck Fast Idle Valve
Faulty EACV (Electronic Air Control Valve)
Throttle cable not adjusted
cruise control cable stuck/misadjusted (not really likely, but I've seen someone try to remove the CC and only got halfway before putting it back in and the cable was stuck)

also make sure the cooling system is bled. Air pockets in the cooling system can cause the FIV to go crazy sometimes.