We have a VB.NET Windows Forms Solution developed using the 3.5 framework. 

The solution has more than 30 projects (dlls) that are referenced from one to another. We even have some projects separated into N layers too (one for the data Access, other for the business logic, and so on).

Since a time ago the solution does not work properly. When we start Visual Studio (2008), it shows hundreds of errors. The vast majority are: 

"Type 'XXXX' is not defined. 

And after doing some "clean" + several "build" we are able to get the binaries. 

Anyone has any idea about what is the problem and how to solve it??