Momentary crappy idle on cold start?

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1998 GS400, 206k miles. When I start my car first thing in the morning, it'll fire up right away, settle at 1200rpm for a second, then slowly dip to around 1000-900 and stay there for about five seconds before returning to 1200.

Every so often, usually when it's colder, it'll start up and then bog down really bad and almost stall, lots of misfiring, etc. Then after a few seconds it's totally fine as if nothing had happened.

No CEL or stored codes. Fresh fuel injectors. No vacuum leaks. All OBD scan data looks perfect, but I've never been able to catch it in the act.

My first thoughts are a coil pack or VVTi solenoid on the way out. Is there anything I can do to diagnose it before something fails?

Your knowledge is greatly appreciated.