stereo equip can drain but usually not that fast
bad cell in the battery maybe? bad connecting wire? have the battery tested if you haven't already. have the alt tested if you haven't already. i know they are new but nothing is ever perfect.
i take it since the battery is new you used a higher powered battery for your rebuild (honestly, i don't know if you needed to or not but just a thought)
those are just my ideas.
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if you can only race me in a straight line then whats the point 
...91 SMF SCCA
http://www.flickr.com/photos/8341770@N07/5601083432/http://www.flickr.com/photos/8341770@N07/5601083432/ by http://www.flickr.com/people/8341770@N07/, on Flickr
I see people out there spending money on the car to improve it … If you spend half that effort just working on your driving skills, making you and the car work together as a team, then you’ll go faster – period. To make the car go a second faster on the track takes a lot more energy and money than making yourself drive it one second faster.
– the late Tom Thrash SCCA Solo E Production