lan problems

Hey all, not sure where my problem lies, hope someone can give me a
pointer.

I have a small lan with various machines connected up to hub with an
ADSL router with DHCP. It connects up no problems and works fine for
about 1-2 hours then the whole LAN loses sight of the router. The only
fix is to reboot the router (read: yank the power cable and reinsert).

My feeling is that MTUs might have something to do with this?

The router (with IP 192.168.1.1) sets up this config:
device add etherb ether //bridge mtu 1500 192.168.1.1
device add ppp_device ether //ppp/DEVICE=1 mtu 1492 151.37.39.27

And doing some ping tests I get the following results:
$ ping -s 1500 -c 3 192.168.1.1
PING 192.168.1.1 (192.168.1.1) 1500(1528) bytes of data.

--- 192.168.1.1 ping statistics ---
3 packets transmitted, 0 received, 100% packet loss, time 1999ms

$ ping -s 1400 -c 3 192.168.1.1
PING 192.168.1.1 (192.168.1.1) 1400(1428) bytes of data.

--- 192.168.1.1 ping statistics ---
3 packets transmitted, 0 received, 100% packet loss, time 2000ms

$ ping -s 1000 -c 3 192.168.1.1
PING 192.168.1.1 (192.168.1.1) 1000(1028) bytes of data.

--- 192.168.1.1 ping statistics ---
3 packets transmitted, 0 received, 100% packet loss, time 2010ms

$ ping -s 500 -c 3 192.168.1.1
PING 192.168.1.1 (192.168.1.1) 500(528) bytes of data.
508 bytes from 192.168.1.1: icmp_seq=1 ttl=30 time=4.79 ms
508 bytes from 192.168.1.1: icmp_seq=2 ttl=30 time=2.19 ms
508 bytes from 192.168.1.1: icmp_seq=3 ttl=30 time=2.19 ms

--- 192.168.1.1 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2006ms

and after some trial/error found that -s 523 is the cut off value.

But, 10mins later and:
$ ping -s 1400 -c 3 192.168.1.1
PING 192.168.1.1 (192.168.1.1) 1400(1428) bytes of data.
1408 bytes from 192.168.1.1: icmp_seq=1 ttl=30 time=4.90 ms
1408 bytes from 192.168.1.1: icmp_seq=2 ttl=30 time=4.87 ms
1408 bytes from 192.168.1.1: icmp_seq=2 ttl=30 time=4.91 ms

--- 192.168.1.1 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 1000ms

So now they are getting through! But then just another 5mins later:
$ ping -s 1500 -c 3 192.168.1.1
PING 192.168.1.1 (192.168.1.1) 1500(1528) bytes of data.
WARNING: failed to install socket filter
: Protocol not available

--- 192.168.1.1 ping statistics ---
3 packets transmitted, 0 received, 100% packet loss, time 2005ms

and again no pings would get through until -s 524 this time.

No idea what is going on here. Any hints would be great!

Thanks,

Marko