Hi there, first post, have used the search function and cannot find a solution.
I have an 03 350z, had warms start issues, cleaned throttle body, and issue was resolved, except now on cold starts the car stumbles and fluctuates between 800-1200rpm for about 5-10seconds then idles normal. It will only do this after sitting for 6+ hrs. If I shut the car off after the 5-10 seconds and start it again it starts normal.
I have never had this issue until I cleaned the throttle body. I have done the relearn through NDS2, as well as the pedal dance. I have replaced the throttle body and gasket, verified correct operation of the coolant temp sensor, verified there are no vacuum leaks, and have also replaced cam sensors, valve covers + gaskets, plugs, and coils within the last year. I have compression tested the engine, and checked vacuum reading while running and both are within FSM specifications. I don't know what else to check, there is no CEL, and not DTCs stored. Does anyone have any input or have you had a similar issue?

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You said you never had the issue until you cleaned the TB, so I would start there. Otherwise - random guesses... Fuel pump? MAF sensor? You might be better off taking it somewhere instead of continuing to throw parts at it.