further thoughts...
4S-18650 would add up to 16.8V... your ballast should be OK with that. But that's not a given by any means.
Heres a short vid of a 35W ballast that draws over 60W (5.35A) on a cold start. Thats a difference of 25W, or a 71% increase over steady-state. AFAIK all automotive HID ballasts behave this way to some degree.
http://www.youtube.com/watch?v=1CSeQCVPCHk
75W / 12.5V = 6A at steady state for your build
It wouldn't surprise me if your ballast is drawing ~100-105W on a cold start though.
when you split that between the parallel cells thats over 50 Watts from each 4-Series cell arrangement. Factoring in voltage sag the 16.8V will drop to ~14.8V
50W / 14.8V = 3.37A
3.37A is a pretty hefty current draw for an inferior 18650. I would contact monster flashlight and find out what current their protection circuit trips at, I would also try and find out what raw cell they use... Sanyo, sony, panasonic (etc). With 8 cells thats a lot of opportunity for at least one to trip. I would think even if one cell trips, that would disconnect its 4S array so all 100W will be dumped on the second 4A array and the current inrush would trip that set as well... resulting in 0V.