When I crank my truck for the first time in the morning, the rpm's bounce like something inside the transmission is randomly putting a load on the engine. It does this in all gears including neutral until the engine warms up and then disappears entirely and drives normally. I don't think it's engine related (I could be wrong), vehicle isn't throwing any codes, services are all up to date, and it runs great except for this weird behavior. Vehicle is a 2001 Toyota Tundra w/ V8 and automatic.
Sounds like you either need to clean your throttle body, or there is an issue with the Idle Air Control Valve (IACV). When the IACV is responsible for maintaining idle RPM. When the engine is cold, the idle is higher, so the IACV needs to open more. It is not uncommon for the port to become partially clogged.