So this hapened on my current and last R too. When the PCV valve and pipework was removed and the idle speed adjusted accordingly, every now and again while approaching a junction the revs would drop to about 500rpm then bounce back to 1000rpm.
It got me thinking that the PCV system did more than suck vapour out of the engine while at idle; it was actually part of the idle control system.
If you think about it, when at idle the throttle plates were bypassed by the PCV system and metered air was introduced from the inlet tube via the cam cover. When removed this makes the idle control valve work a bit harder to maintain 1000rpm idle speed.
So what ive now done is make up a vacuum manifold across the old breather ports on the inlet manifold (similar to the mighty mods dump valve setup), connected to a simple check valve and back to the small connection on the inlet tube. The engine can now suck through this "manifold" at idle and when under boost the valve closes to stop boost escaping.
For the past couple of weeks ive been running this the revs havent crashed once and generally seems to be happier at idle. All for the princely sum of £4 for a 6mm one way/check valve!
Only wish id thought of it sooner...
I can post a link to the valve ive got if anyones interested?
Cheers for now