Had this Previa since new. Has 220k on it. Has always had a rock steady idle of 600 rpm with a hot engine. Recently, however, it has developed some quirks. Firstly, the idle speed would periodically drop to 400 rpm. Next it would occasionally idle at a steady 800 rpm. Now, in addition, it will sometimes pulse between 600 and 800, and between 400 and 600. These changes have happened over a period of about 4 months. It still drives totally fine, but with the number of rebellious idle behaviors increasing I worry the next one will be more troublesome, especially if it decides it wants to drop lower than 400.
Any ideas where I might start looking for culprits?