Hi, 6 years is a good age for a battery, its bound to be feeling its age. If you have a battery charger, couple it up and see if it shows a high charging rate just after the problem occurs, and this indicates your battery is low or flat. Otherwise have your battery tested by a reliable Garage. I had the same trouble during a very cold spell recently, I charged the battery and have had no trouble since but my battery was on the car when I bought it so I dont know how old it is.