Hi volvo wizards - another little puzzle for you:

My amazon wagon (b18 bored to b20, K cam) runs like a top after I put on a set of fresh carbs from Rhys at Island Automotion (thanks again Rhys), but one thing that I was hoping would be a throttle shaft problem, now seems to be something else. When the car warms up and is driven at 3000rpm plus for any length of time, and then stopped, the idle jumps up around 2-300 rpm and sits there. Eventually, after 5 minutes or so, it wanders back down to where I have set it. The car has a pertronix conversion installed, the vacuum retard on the distributor is disconnected, and the advance seems to be working as I can run the timing between around 10 and 30 btdc on my timing light by revving her up. Another clue is that if I turn her off when idling high and start back up again immediately, the idle is back where it belongs. The throttle linkage is definitely not binding either (I can pop the hood and see it completely at rest). It seems like maybe the centrifugal advance is sticking, but only sometimes (?).. let me know if you need any more info.

Any suggestions / thoughts / experience would be most appreciated.

Dan