1991 850i automatic...low idle.

[Tomcat BMW]

My 850 runs smooth at idle and through the complete rpm range...EXCEPT, I have noted a smooth (but a little low) idle (about 500 rpms) while in park/neutral BUT when I sit with the brakes on with the transmission in drive the rpm seems to drop to ~300 rpm (which makes the engine sound/feel rough). Is there a mechanical way (or electronic way) to bump it about 200-250 rpms...or compensate for the transmission in gear by "automatically" raising the rpms.
Thanks,
Tom

[Tomcat BMW]

I'm going to try PCVs as DKs are rebuilt (and were running/idling fine).