Mitch,
It's a safe assumption that you are drawing WAY to much current from that pack. no 1200Mah pack can deliver what a 2000Mah pack can deliver.
If you don't have a watt meter, now's the time to invest in one.
You did not mention the "C" rating of the battery pack, if it's a good C15 rated pack, drawing anywhere around 15A will beat up the pack in short order.
Good luck,
Pete