Sounds like either the setting for the idle air screw is set too high (what is the idle speed in neutral at normal operating temp?), or the auxiliary air valve is not closing properly during warm-up. Prompt cold start-up implies that the cold start valve (adds extra fuel) is fine.

See if closing down the idle air screw improves the surging. If the surge disappears but the warm idle is then too slow, I would say that AAV is very likely the culprit. Should be fully closed at ~70ºC, you can check by disconnecting the air hose to this valve and checking for suction.

Excellent info on the AAV (also used in D-Jet Jags):

http://www.jag-lovers.org/xj-s/book/AAV.html
http://www.jag-lovers.org/xj-s/book/AAVdisassembly.html

I rebuilt my AAV by disassembling, cleaning, and adding a shim to the thrust piece attached to the plunger on the themostatic bulb. Sometimes the upper half of the AAV cylinder has come out of the base a few mm and you can use a mallet to push it back and restore proper closure behavior.