there are several solutions/suggestions here at the Plesk - Forum and over the Odin/Parallels - Knwoledge-Base. If you experience any issues/problems, please describe your issue/problem and let us know your operating system, your Plesk version ( incl. MUs ) and probably add log - files and/or configuration files, so we can help you with investigations and suggestions.
Ehm, I paid for an "optimized computing" solution like Plesk to have it done by plesk and no listing of configuration files, I can adjust by myself?! However I'm afraid that won't set dhparams for all existing and future sites?
well thats a solution for the vhosts, but what about the Plesk Panel itself?
This server supports weak Diffie-Hellman (DH) key exchange parameters - by Qualys SSL Report