I've an Olimex Lime 2 and I also have freezing problems.
The setup is Olimex Lime2 with SATA-drive connected (drive is powered with additional power supply), console cable connected and network disconnected.
The board freezes when i run a task, that does a lot of reads on the filesystem and there are some big files involved. It freezes sometime during the task. There is no console output that gives any hint.
I've tried different power supplies, with or without battery pack. Same problem occurs, when I connect USB-drive instead of SATA-drive.
I've tried different u-boot/kernel combinations (armbian, olimex, debian installer).
I've tried different settings for "frequency switching" as described here.
The only stable running system is with "a20-lime2_debian_3.4.90_release_2".
This is base on sunxi-uboot and sunxi-kernel.
Any ideas?