But to get into this your best diagnostic and to get more ideas from forums is.... Get the OBD codes read.
My view has changed on this over the years. That is, I want all the codes cleaned up. I find that most of the time when I fix what causes the codes, the problems go with them.
I have a 2002 6-cylinder Lexus RX 300 that is idling too fast. When the car starts, the idle works fine and breaks down, however, as I drive it changes. The engine light is on and sometimes blinks. The valve pan cover is also leaking oil. I changed the front part of the idle air control and all but one of the ignition coils. I also changed 1 spark plug recently and I did an oil change just over a week ago. The idle sensor was also recently changed.