Greetings, hate to drag an old thread up, but My 2000 LS400 have this exact issue. The car is new to me and is a choice example, all maint records (dealer maintained, garage kept). Very odd problem, cyclic or ON-off-ON, like a waveform. Vibration, doesn't feel like a miss. Only on cold start up, gone when up to operating temp. Dead smooth in fact.
I would like to hear from the OP what was the fix??
Thanks
Andrew