from the way you describe this, the starter solenoid is doing the clicking. It will do this when the battery voltage gets low. check the voltage of the battery, it should be about 12.5 volts when the unit is off. when it is running, it should be around 14 volts.
either charge the battery or bring the battery in and have it load tested. if it is good, the charging system may be the issue. Also keep in mind to check the grounds. remove the bolts to the solenoid and then reinstall, this helps with a ground issue.
