Entity Framework Code First, Unit of Work, Repository and shared DbContext

I'm building a web app using EF Code First and ASP.NET MVC. I have following types:

IProblemRepository
EFProblemRepository
ICategoryRepository
EFCategoryRepository
CleanStreets // db context
IUnitOfWork
// etc.

Code snippets :

public interface IUnitOfWork
{
    void Save();
}

public class CleanStreets : DbContext, IUnitOfWork
{
    public DbSet<User> Users { get; set; }
    public DbSet<Role> Roles { get; set; }
    public DbSet<Point> Points { get; set; }
    public DbSet<Rating> Ratings { get; set; }
    public DbSet<Picture> Pictures { get; set; }
    public DbSet<Problem> Problems { get; set; }
    public DbSet<Comment> Comments { get; set; }
    public DbSet<Category> Categories { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<User>()
                    .HasMany(u => u.Comments)
                    .WithRequired(c => c.User)
                    .HasForeignKey(c => c.UserID)
                    .WillCascadeOnDelete(false);

        base.OnModelCreating(modelBuilder);
    }

    public void Save()
    {
        this.SaveChanges();
    }
}


public class EFProblemRepository : IProblemRepository
{
    private readonly CleanStreets data;

    public EFProblemRepository(CleanStreets data)
    {
        this.data = data;
    }        

    public void Save(Problem problem)
    {
        if (problem.ProblemID == 0)
        {
            data.Problems.Add(problem);
        }

        data.Save();
    }
    ...
}

At first, I didn't have a UnitOfWork. I created a new context in every repository. But after I wanted to save a Problem (Problem includes Category), using the Save method provided above, I received the following error:

An entity object cannot be referenced by multiple instances of IEntityChangeTracker

I found, on stackoverflow, that the problem is with my db context and the solution was to create a shared context with the unit of work pattern. I tried to do that (as you can see above) but I still get the error. Every time when I want to store a Problem the error pops. Did I implement a "shared" db context right?