Too large buffer with External MP3 player !!!

Hi

I use Slimserver with some Slimboxes and also VLC software and for zones Rotel RDG-1520 (Radio and Internet Tuner). On zones using VLC or Slimbox, when I change of track on server, it's no more than 1/2s to get music to change. On zones using the Rotel, it takes up to 30s (still playing previous track during the 30s). Is there a way to setup in Slimserver smaller buffer for these zones ? or is it something to setup on Rotel ?

Thanks for help and ideas

Vincèn