Legal question regarding slander?

I'm not sure if the actual action involved is slander, but I would like to see what sort of recourse I would have. A former employer has told prospective employers in times past some information about my time employed with them, and it has cost me 2 decent paying jobs. The last time it happened was 2 months ago, and I was actually told from the last prospective employer was actually quite kind and told me what it is exactly they had said about my character. Is that slander, and if so, what can one do about that legally? What would I have to prove to win a case?