ADSL router - can't connect to internet

[fingal]

Hello - I've developed a nasty ADSL internet connection
problem which goes as follows. I can't connect to the outside
internet, but I can ping my internal network/router.
It's a desktop box with an IP address of 192.168.1.1,
and I connect used a wired D-Link DSL-504T router and
a 3-Com ethernet card, which has been extremely reliable to date.

Mandriva reports that my network is up on eth0 (in fact this
is obviously so) but I can no longer ping any external web
address. Indeed, my router won't connect at all, where
previously I had no trouble. My router's log file contains
the following info., repeated over and over. The date
is nonsense and is (I think) a bug. I know the correct
date because I've been clearing and checking the
log just to make sure.

Jan  1 05:33:13> pppd 2.4.1 started by root, uid 0
Jan  1 05:33:13> Connect: ppp0 {--} 
Jan  1 05:33:16> CHAP authentication failed
Jan  1 05:33:16> Connection terminated.
Jan  1 05:33:16> Failed to Authenticate

I gather that CHAP is a Microsoft 'handshake
authentification protocol' which I found out about
accidentally while looking at other routers with a
view to replacing this one (assuming it's faulty).
I should explain that my net connection has been
getting increasingly unreliable, disconnecting me
from time to time without warning.

I learned that if I restarted my router I could
connect again with few problems. Something has
started to go wrong, and it's got progressively
worse until I'm completely off-line at home. At
first I thought it was my ISP's fault but not now.

Is my router really due for replacement? I dismantled
it and had a look, but nothing is visably wrong.
It's just a single board, and it looks fine but this
could be deceptive. I thought I might see a
burn-out on the surface of the board, or a loose
wire, but no.

Some techie stuff about my routers' settings
which may be useful:

Uses PPPoA
MTU=1500 (though it defaults to 1400 ... I prefer to set
it to 1500. Mandriva is using the same MTU)
MRU=1492
NAT enabled / firewall enabled
VPI=0
VCI=38
QoS: UBR (I have no idea what this means!)
Modulation type: GDMT
Commects using DCHP

As far as I know these settings are okay, but any
advice, guidance or information gratefully received.
Even rebooting the router does no good now.
The router looks pretty innocent, blinking away
on my desktop ... Nothing has changed outwardly,
but it's funny how fragile a net connection can be!

Note to moderator(s): Maybe this should have gone in the networking section? I'm not sure considering that I think my router might be faulty.

[fingal]

Well ... I got it working again. It's not very reliable for some reason (a bit like an arthritic donkey on valium) but at least I can connect. The trick was to ignore the manual and use a combination of Google for further information, my brain and sheer bloody minded determination. So there you have it. Don't read the manual!

The other thing which I found to be useful is 'power cycling'. So if you find you can't connect, just pull the plug out of the back of your router and then plug it in again. Your router will restart and perhaps ... just perhaps ... you'll get your connection back again.