1999 Ford Fiesta not starting?

My car is on it's 4th new battery. It has been serviced only recently and got a new coil pack, plugs and battery terminals have also been changed. My problem is my car can run for 3 days or few months without any problems, then it will refuse to start needing a jump start can be fine all day or after a long run then can cause the same problem again. Is very unpredictable. If left for running or not running can start shortly afterwards no problem. Is making ticking noise and key is turning when won't turn over. My mechanic says can't be starter or altenator if their is cuccess with jump start or no lights are appearing on the dash showing any warnings for battery. Both of us at wits end my first car have it less than a year. My boyfriend also a mechanic thinks their might be something electronic draining the battery. Any help greatly appreiciated?