I suspected an inlet leak when one of my cars did what you described but it turned out to be the ISCV. Inlet leaks can disappear once everything expands when hot so that's the way my mind went, with a 500 there are a few places for it to do that I guess.

ISCV was cleaned by specialists Promax (Geoff Dewitt's old team) and the nut loc tighted back on but I didn't trust it so replaced it (didn't want anything falling off and into the engine). It did make a world of difference but as Tab says sensor could be sending the CO2 all over the place.

Am still running the replacement ISCV 7 years later and the problem has never come back... but its a cossie so one day it will - lol