Hard Start

My 300E has been acting weird latley. Every once in a while it will not start. this normally happens when I stop somewhere for a few minutes. When it first did it I let the car sit overnight and it fired right up. The second time it would not start, I removed the leads to the coolant sensor and inserted a wire between them. the car started right up. I figured I had a bad coolant sensor so I purchased another frm the local MB Dealer. THe car did fine for a couple of weeks and then began to not start as before. I have learned to short the 2 terminals when ever this happens and the car starts right away. (Not Stranded) Has anyone had anything similar happen. I was wondering if the overvoltage protection relay could cause these symptoms. I can not believe the coolant sensor has gon bad that quickly. Any help greatly appreciated