without stalling at other times, or a longer stall here's my best guess:

Your camshaft position sensor

You say your indy shop checked codes and nothing came up, do they have a general scanner or volvo specific? Allegedly your computer picks up on your electronics not working and will trip a code even if the CEL doesn't come on. A ECM-3000 or 3010 showing up would explain your problem. You could probably get the dealer to read your volvo specific codes and confirm or refute before you spend the money on a new sensor.. Just talk with their parts guy and ask if they would read for free, i know the one out here does.

But without that, Below is a link to where the CPS is on your car. You might want to pull it out and check the connection. I believe it works on induced current so there won't be too much for you to damage if you use brake cleaner, but I'm not actually sure on your car. (THAT MEANS USE CAUTION...) :-)

Hopefully thats all she needs, but you might have to replace it.

http://farm5.static.flickr.com/4082/4797457802_ea92f78d28_b.jpg