Idleing too fast

Hi all,

I have one of the infamous zd30 patrols (auto), over 120 000k and generally going well. However I have noticed in the last week that the revs are creeping up from the usual 750 at idle to around 10000 - 12000. This happens when I put it in park.
Its ok when I first start up, just happens later after I have been driving for a while. I have checked the fault codes, but are all clear.

Any suggestions what might cause this?

Thanks
PJ.