12v Halogens, transformer connector's melting?????

Hi, I have 2 sets of 4 halogens(50w) in my apartment all with there own transformers.. after six months i noticed that the plastic connectors on the transformer had melted and almost caught fire, when i checked all the transformers were the same.. so i changed all the transformers for new ones and now 6 months later the same thing.. its always the output connection to the bulb that burns out.. Oh also they are on a dimmer if that helps.. Any help would be greatly appreciated.