Oxygen sensor problem?
 

Hi, my 1996 ls400 (97000 miles) occasionally seems to idle too low ie below 200rpm and then stalls. When it does this it runs rich where black smoke comes out.

However when i restart it, it starts up fine and revs freely. And then it idles normally for a few minutes before dropping again into low idle.

Driving around is fine and power seems to be normal.

There's no check engine light or anything. Could this be a sign that the oxygen sensors need replacing?

I don't think it's an oxygen sensor problem. The ECU detects failed oxygen sensor checking the output voltage and the time interval of it and the engine check light is turned on.

Judging from your symptoms, I think your problem may be caused by the clogged or failed IACV.

+1 Have you ever had the throttle body pulled out and cleaned? If not, it is probably full of gunk.

yes, clean throttle body as Lavrishevo mentions, or at least the butterfly portion, if that doesn't fix the idle the then it is almost always the Idle Air Control Valve (IACV) bearings as Yamae mentioned or else failing ECU capacitors

there is a test in the service manual to verify correct IACV operation - you apply battery power to the correct pins and watch the plunger move - there is also an integrity test of the IACV coils