stallout?

I have an imperial stout brewing with an OG of 1.09. I
m using Wyeast 1056 that was in a starter before I pitched it. Here's the situation. Things were looking great. At 8 hours after pitching the yeast they were really active, foaming, the fermentation lock is going big-time, etc. I had a friend over to watch the game yesterday and showed him the fermentation ( in a 6-gal carboy) and then he left. I grabbed a homebrew from the fridge, walked back into the room where the carboy was and it was dead dog silent -- nothing going on. It wasn't a case of the yeast slowing down -- they went from wildly active to nothing w/in 10 minutes. I had heard about yeast stallout so I swirled it a bit, but no significant results. It has a really thick layer of krausen -- probably three inches. At the very bottom layer of the krausen there are a lot of bubbles. The fermentation lock has now been quiet for 12 hours. Is it possible that the krausen layer is so deep and thick -- it is really deep and thick -- that it has the CO2 trapped beneath it and that is why there are no bubbles in the fermentation lock? Also, the bubbles quit coming 18 hours after the yeast was pitched. So it was active for 10 hours, and then instantly nothing. Strange! Any solutions you guys can see? I would think that the yeast died out except that it happened so quickly! I have a packet of Danstar Nottingham ale yeast and a packet of Saflager S-23 lager yeast -- I could use one of those for additional pitching, but I don't want to mix them with the 1056 if there is some other problem. Thanks

A venting air-lock means nothing; if you have a krausen, fermentation is still going on. How large of a starter did you make for that beer? 1.090 OG is going to need a sizeable starter.

Read this...http://blogs.homebrewtalk.com/Revvy/Think_evaluation_before_action/

Don't try to fix something based on your airlock..leave it alone for awhile...if you have krauzen you have fermentation.