Semi-functional net connection through LAN

[Golar]

I come back from Israel for 6 weeks and my family greets me with a semi-functional network.

My setup: 4 computers on a LAN through a Umax Ugate 3000 router to share a cable modem.

The problem: 1 computer's internet connection is slower than dialup when going through the router, but when connected to the cable modem directly its fine. Meanwhile, another computer connects to the internet at expectable speeds through the router.

I've tried different cables, different ports on the router (it also serves as a switch), and updating the firmware.

Any ideas?