T500 shutting off when on battery doing GPU task

[hoax32]

I got this beautiful T500 brand new out of the box infront of me.
Got it from a company the never got a chance to use it.
The battery was dead from sitting around for 4.5 years.
I bought a knock off battery off of ebay which holds a nice charge.

But whenever I start something GPU intensive, it randomly crashes.
No Blue Screen - nothing.
The screen goes black and 1 second later the LED's turn off. (Like the lest second when you shut it down)

Everything works fine with a power cord though.
Any idea what's the problem?

Forgot to mention - the HD3650 is heavily overclocked. (No stability issues when plugged in)
Clock is %33 OCed and Mem is %35 OCed.

Temps are all good capping at 84*C with the GPU.
Hardware test all passed.