Sounds sort of similar to mine, except mine doesn't work it's way down in stages (no pauses every 50rpm), it just slowly lowers until it hits 550 when very warm (500 can occur only on a damn hot day and after an hour drive or longer).
You didn't mention any pausing when it lowers down to 550 but that was the impression I got from your post.
750-800rpm is "too high" for an M35. The factory idle is set at 650 +/- 50rpm.
Mine has been doing this for about 2.5 years now with no ill effects.
In that time, I have added/upgraded/replaced - turbo, radiator, intercooler & piping, thermostat, runner spacer, throttle Body, dump pipe, frontpipe/cat, cat-back. None of which have made a difference to the lower than normal idle.
I have not changed my PCV as it is still working properly it would seem.