The AT91SAM9260 CPU only has a small on-chip buffer for the Ethernet port. The TX under run may occur under heavy network traffic. The patch that uses on-chip SRAM as buffer suppose to fix the problem. Make sure you have the following configuration for your kernel build,
Ethernet (10 or 100Mbit) --->
<*> Atmel MACB support
- Atmel MACB TX buffers in internal SRAM