Have you reset your ECU at anytime? It's worth trying...

It's actually referred to as valve lash or valve adjustment - FYI.

If this hasn't been done in a while, I'd do that too. I really don't think it would have much to do with your idle though.

Vacuum lines are all over your motor. If you hear a hissing, you most likely have a leak. Also check the lines for cracks or brittleness. It could be that one has a crack and the leak only occurs sometimes giving you the odd idle only sometimes.

hih