And as mentioned, it would also be worth it to check the intake manifold bolts. They do have a tendency of getting loose.
This is the first thing to check. Cheapest, quickest, least invasive.
The torque spec is fairly light on these bolts, as I recall. Like in 'inch-lbs'. So don't gorilla them.
If you do find one or more loose, it may be useful to put some blue threadlocker on them before tightening.
Don't use red!
This could be the intake manifold gasket. Cold it can shrink, allowing un-metered air in, causing a lean condition. Once the engine warms up, they expand, closing off the leak(s) and restoring normal A/F ratio.
This is what I was thinking, myself. To test, get a can of starting fluid. Brake cleaner can also work.
On a cold engine, raise the hood, and locate where the intake manifold joins the cylinder head(s).
Start the engine, and head back underhood. Spray at the juncture of manifold / head. If you notice the idle smoothing up for a few seconds immediately after spraying, you've found the culprit. Note that leaks can be one or many in nature, in different places. So you may have to spray in a few different places around the manifold.