My '89 780 with B230FT (auto) has been giving me a problem lately that I just can't figure out. When coasting along in drive the vacuum gauge shows more vacuum than my other two turbo volvos show (like all the way to the left on the gauge). When I begin to come to a stop, the idle will drop down to about 500, the vacuum/turbo gauge will jump up to where it normally sits on my other cars for 20mmHg and the idle will come back up. My problem is that sometimes it doesn't catch the problem early enough and the car will just stall out. It is really driving me nuts, b/c this is certainly a danger when pulling off of exit ramps and other places when there are cars behind me if my car stalls out when stopping.

The car idles smoothly in park or neutral at 700RPM and doesn't change when the brakes are applied when standing still. The car has normal pick-up and power. The car boosts normally. I have already replaced all of the vacuum lines in the car.

What am I missing?
Thanks