What ESC does the Slash have?
What are you using to charge your batteries?

Using LiPo batteries is not a necessity, but they will offer better performance. Perhaps your ESC is setup to use LiPo packs and the LVC (low volage cutoff) needs to be turned off while using NiMH packs. Weak cells, batteries not fully charged, and bad solder joint on the wiring/adapters could be issue as well.