IIS7: A process serving application pool ‘YYYYY’ suffered a fatal communication error with the Windows Process Activation Service

That looks like the IIS worker process is crashing. If you have multiple sites/applications running on the same AppPool, I'd start by isolating them one at a time into separate AppPools. You'll probably find which one is causing the problem that way, which should give you a place to start looking.

Aside from this, I'd guess I'd take a look at using adplus to get a crash dump of the AppPool process and then run through it in WinDBG to see what comes up. There are some good resources for this out there. Two that should help you get started are this and this. They are a bit outdated but the basic mechanics should still be mostly the same.

This one is a pretty nasty one. I recommend getting the IIS Debug Diagnostics tools. You can probably alleviate the symptom by turning off "Rapid Fail Protection" in the properties of your application pool, but that will just mask the problem. What is going on is that your application is experiencing an exception so often/so repeatedly that the service shuts itself off. You'll need to run the diagnostic tool to capture everything going in and out to find the exception that's occuring.

This can be a very laborious task because the tool will gather gigabytes of junk to sift through. If you're lucky, the turning off of the "Rapid Fail Protection" might allow the exception to proceed through to the browser, but my guess is that the server will just end up hanging.