Originally Posted by Jordo!

It shouldn't.

I'd look for a rupture along the intake tract itself -- I had stupidly over-tightened the hose clamp on my old Z and managed to bend in slightly the neck of the intake box where it meets the tube causing an intermittent leak and weird idle.

Another possibility is a failing header gasket or problem with the TB's (apparently, not uncommon) or a small pinhole or crack in one of the resonators attached to the intake tubes (I had one of those crack where it clamped to the intake hose).

The only other things I can think of are something screwy with the misfire detection logic, (but the high idle definitely sounds like a vac leak) or a dirty MAF sensor giving misreads.

If you can't pin it down mechanically, I guess you could try doing so via datalogging if you have Osiris or if you can get hold of OBD II cable and software.