Decrease performance when ASP.NET 3.5 installed on IIS 7.5 from IIS 6

Hello all,
when I've installed my ASP.NET 3.5 intranet web application on IIS 7.5 - having the same database (SQL Server 2008) -
the performance have falled down, and appear some connection or database errors that first appear like warnings.
Where and what I have to investigate?

Thanks in advance.

Luis