The needle on my RPM gauge bounces up and down and my engine "studders" as my car is decelerating from higher speeds. I pull over and turn off the engine for a just few minutes, and it starts up nicely and runs smoothly, no problems at all (as long as I don't go over 60 mph). It sounds crazy, but I swear that on rainy days or when humidity is higher I can only get to 50 mph before it acts up. No check engine light ever comes on when this problem arises. I've had 4 oxygen sensors and spark plugs replaced so far. I have 105,000 miles.
My engine idles rough after driving at higher speeds. (50+ mph). on 2000 Toyota Solara
The idle speed is controlled by an idle air control motor on the bottom of the throttle housing, they do "carbon up" but usually cause stalling when started from cold. I would look for low coolant level, perhaps a vacuum leak, or just bad electrical connections in the engine compartment. With the engine running wiggle wiring harnesses in the engine compartment and see if the engine idle speed changes or the engine starts to run rough.