If the ground strap from Trans to chassis is not clean, then chances are, it will still start with a jump pack, however; the bad earth connection will only get worse, and in extreme cases can get very hot due to the resistance build up between the ground strap, and chassis, remove the ground strap, clean it at the trans end, and chassis connection thoroughly, use some copper grease compound when re-installing both ends, that should sort it!
Ant.