ATI Radeon HD 5970 and HD4870X2 problem SOLVED!

@Garnet_Alexandria: You are right, I remember a friend of mine burning his graphics board with a 2D classic game... Anyway, in my case you were right in your first post, it was a multi-GPU problem.

@Nichaedemus:
BINGO !
I took your advice and disabled the secondary GPU and now the game runs as it should: smooth and beautiful.

Thank you, thank you, thank you !