I observed this behaviour when my router was connected to the DSL modem via a 100 Mbit USB to Ethernet adapter by DLink. Replaced that with an Intel based PCIe NIC and that solved the problem for good.
You are obviously connecting through a real NIC. I would only recommend replacing Realtek with Intel, but it is unlikely that is your issue. Dunno, sounds odd.
You do not need anywhere near 2 GB RAM. I am running with 256 MB just fine and ZS only peaks around 120 MB. My router has 1.6 GHz Atom now, previously it had a 2.2 GHz Celeron P4. So your hardware should be adequate to the downlink you have.