Dramatic, slow, classical, violin music?

The earlier mentioned/linked piece by Samuel Barber is a good choice. It was once referred to as the saddest (most heart-wrenching) piece of music in the genre.

Here's one of my favorites from one of my favorite composers, Antonin Dvorak, his Romance for Violin and Orchestra:

http://www.youtube.com/watch?v=0dFHuCXqvx4

Here is the middle movement of Christian Sinding's Suite in A Minor, the Adagio. Note when the Viola intertwines with the Violin at the 2:55 point:

http://www.youtube.com/watch?v=Lk3z0RB5r5Q

And Danse Macabre by Camille Saint-Saëns, although it may be a bit faster than you intend:

http://www.youtube.com/watch?v=YyknBTm_YyM

And, on edit, a piece I'd not heard for YEARS. Vitali's Chaconne in G minor:

https://www.youtube.com/watch?v=BikbXFPoN28

Slow Violin Music

http://www.youtube.com/watch?v=izQsgE0L450