I have a 2007 c230 that would do the same thing but it also stopped running all together a couple times and caused the engine to start overheating. It ended up being a faulty fan assembly. The fan and fan module are sold as one unit and when the module fails you have to buy the fan assembly. There is no relay for the fan, it is contrlled by the cars electronics feeding information to the fan module. So you will need to have it checked to verify if the fan is actually the cause.