I just want to share my experience with my newly flashed Zyxel P-660R-D1 modem. I flashed it with the new firmware from Zyxel - 340AGE3C0.bin. It works great and stable too. I am using the modem as a modem router with a Linksys WRT54G as a switch with wireless access.

Just to be safe, make a backup of your original PLDT firmware.