Your battery voltage is too low when under load. What is happening is the starter engages, the battery voltage reduces due to the starter current - to the point where the solenoid has insufficient power to hold it 'in', so starter disengages, voltage rises, and the cycle repeats.
Check all connections to the battery, and check engine earth cable. If all OK, replace the battery.
phil_saunders@bigfoot.com
Phil Saunders, October 2008