When cold, the idle speed may start high (at around 2000rpm), then drop to a more normal 800rpm.
However, fluctuations between 2K and 800 rpm 'base' idle speed, is known as idle 'bouncing'. That is, it drops to 800, then the car revs for a moment until the rpm is around 2200, then it drops back, and repeats.
This can be caused by bubbles in the cooling system traveling past the temperature sensor causing incorrect readings. First-order fix: bleed the cooling system.
An additional cause may be a leak in the water pump, causing the bubbles. Fix: replace water pump, bleed cooling system.
Yet another can be a kink in the coolant line feeding the Idle Air Control Valve (IACV) on the throttle body so that it doesn't get the proper signal and idles too high. The computer then tries to compensate and it starts bouncing back and forth.