buckaroobanzai,
The behavior you're describing is the default one. But the .NET application can be maintained in memory and never released (or released less often) if you tweak the parameters of the ASP.NET application when declaring it in Abyss Web Server.
More specifically, check Preload and set Request Count Before Process Recycling to 0 to disable process recycling as explained in
http://www.aprelium.com/data/doc/2/abyssws-win-doc-html/hosts-configuration.html#HOSTS-ASPNET