Well, its not just about heat sinks...

The DC Condition, as thedatasheet says, is the maximum rating.. that too per channel. Now, if we drive both channels together @ 2A per channel, for at least a couple of minutes, with a heat sink, its time to buy a new one.

I do respect the claims that might come, but i has been the experience to me.

As for the original question, just measure how much current your gear motor and drive motor takes, separately. see if your power supply(or battery) is rated enough..

Just as Vikas said.. it seems to be a problem with the juice....