This is a quirky but known problem with these trucks. The revealing question is, believe it or not, whether she had the cooling system drained before the problem started. On refilling, there can be air bubbles trapped in the bypass pipes, which are responsible for telling the cold idle control valve whether the engine is at operating temperature or not - it opens to allow extra idle air when it thinks the engine is still cold. When its opinion is wrong - because the trapped air bubbles don't conduct heat the way coolant does - it stays open, and the computer keeps trying to reduce the airflow to compensate, resulting in the pulsation. The test is to feel the little hose going to the valve, after the engine is warmed up: if the hose isn't too hot to touch comfortably, you've found the problem. The fix has nothing to do with anything the guys mentioned. It's to redrain the cooling system and unclog all the coolant hoses and passages that serve the cold idle control valve...there are a BUNCH of them, and some of them are TINY. Then refill the system, burp out the trapped air, and the problem will be gone. It's wise to examine beforehand the hoses you'll be dealing with, and if they're old, pick up new ones from Toyota ahead of time, so you won't be stuck with an undriveable truck in the midst of it all.