It seems that the IIS workflow is crashing. If you have several sites / applications running on the same AppPool, I would start by isolating them one by one into separate AppPools. You will probably find which one is causing the problem in a way that should enable you to start your search.
In addition, I would suggest that I look at using adplus to get a crash dump of the AppPool process and then run it in WinDBG to find out what is happening. There are some good resources for this. Two to get you started, this and this . They are a bit outdated, but the basic mechanics should be basically the same.