1996 Toyota Avalon very high idle when warm

My Avalon idles about 2000rpm for about 30 seconds to 1 minute even when the outside air temp is 70 degrees or above. After only about 2 minutes will the idle drop to below 1000rpm and finally settling at about 750rpm when at operating temp.
I have already changed the coolant temp sensor and have had a mech analyze the system as a whole.
Any suggestions? Or is this idle speed normal?
Thank you.