Hi weoh,
I also had the similar problem as yours. When my DGL-4500 run v1.15, due to I have no computer use wireless, only Wii, PSP, Buffalo Link Theater use 802.11g, NDSL uses 802.11b but all rarely use the networking, so I've tried disable the Wireless configuration. Then I found my DGL4500 reboots automatically every hour. Until I re-enable the Wireless (no matter I choose mixed n/g/b, n/g, g/b or only n, g, b), the router has no more reboot problem.