strange idle while in park. 99 4runner

[hawkcharmer]

last night I was sitting in the truck while it was parked and running. I noticed the idle jump from 700 to 800 pretty consistently. The engine was warm and had been running for about 30 min. It would hold at 700 for about 4-5 seconds then jump up to 800 for another 4-5 seconds, then back down to 700. It kept repeating this pattern. the truck just turned 200,000 miles.

Any ideas what I should look into?
Any feedback is greatly appreciated!
Thank you!

 

[Whitson Metalworks]

silly question, but was your A/C on?