PPP authentication fault

[#1]

For about the last month, using my standard Speedtouch ST585 modem, the modem login process at modem power up has almost always stalled after successful DSL connection.  This has been easily remedied, just by pressing the Connect button in its browser page.  It may then, once or twice, report Disconnected by User, but  then always succeeds in gaining internet connection.
I eventually got round to experimenting with another modem, this time the Speedtouch ST530.  This one has the virtue, which unfortunately my ST585 as currently configured does not, of doing automatic  PPP CHAP retries until success.  As you can see from the attached logs, the single common factor on three consecutive startups has been a failure to authenticate properly on what seems to be the default first host tried, rhost = acc-aln1.l-tow.  Whereas when rhost JUNOS eventually receives it, it  authenticates without problem.
I'll be very interested to know the cause, presumably at the exchange's or PN's end, and why it's not a generally reported occurrence.  Is perhaps my ST585 very unusual in not doing the retries?  I got it as a freebie, and a fair amount of its CLI instruction set is not available to me, even as administrator.  I think there are other, not widely published, privileged user logins available on it.  It would be nice to have one, even if it's not a route to resolving this particular problem.

[#2]

acc-aln1.l-tow indicates some part of the BTWholesale network
JUNOS indicates one of several Plusnet gateways, others might appear with a more specific name such as PTW-AG01
When things work normally, I think you will usually see one log entry with acc-aln1.l-tow and another with a Plusnet gateway name.
I don't know what the problem might be. Most routers would just keep trying, I suppose some might have a limit on the number of retries. Looking at my own router log, I see mine has often needed multiple attempts to re-connect recently.

[#3]

@ejs, thanks for helping revise my mental model.  I'd previously been with Virgin, where authentication seemed to be reported at just one node.  But I see that IPStream has it at two stages.  
So, there's clearly a problem with that particular first node being rather sticky.  Given that your modem has to retry too sometimes, I don't know whether the stickiness is within generally accepted system standards or not.  But it does seem to be  a known correctable fault, as in eg http://forum.kitz.co.uk/index.php/topic,11500.0.html, so I think I ought to ask for it to be checked.  Will someone from Plusnet pick it up from here, or should I put in a parallel fault report?  I've not ventured as far as trying BT test login, if accessible, as described at http://www.kitz.co.uk/adsl/bttest.htm, but am willing to if it might be helpful.
One other oddity that might, or might not, be related.  Intermittently with the ST585, initial syncs over a few months have  occasionally  been in the 2kb/s region rather than the usual 10kb/s. A fresh cold start usually rectifies that.