ALSO - Assuming you have the OEM brushed motor and the OEM ESC = HPI SC15 W = That esc is LiPo compatable according to the manual (my manual anyway).
BUT you have to reset the esc for LiPo use according to the manual FIRST, otherwise you could blow up the battery, etc....
I believe that esc has a low voltage cut off, as well. With my S2S the low voltage cutoff worked on the OEM Ni-MH battery as well as the LiPo. So, it could very well be low battery voltage.
Pushing a Ni-MH battery for 20 minutes is really stretching it.
Read the manual.