Strange and persistent PPP problem

Put simnply, sometimes I can make a successfuk PPP conection with my
ISP, and sometimes I cannot. Moreover, even when successful, after a
period of time ranging from seconds to hours, the connection can
suddenly cease to exist.

For example, today I loggen into a local GNU mirror, and successfully
downloaded over 7MB of the gdb-6.0, and then, suddenly, no connection.
For the next hour or two, no connection was possible.

The PPP is via an analogue modem attached to /dev/ttyS1


Any suggestions? I'm desarate!

More Info:

1. Before moving house, my PPP connection was perfect. After moving the
problems started with the same hardware and software.

2. My ISP (my university) claims that all is well at their end, and have
had no other complaints.

3. Because PPP works sometimes, I deduced that the problem may be
hardware. But since moving house, I now have a new computer, many
new kernels, latest ppp, a new telephone plug, and even a new cable!
I even bought a new modem, but when the same problems occurred, I
took it back to the shop.

4. This may not be relevant, but when, as now, I download something and
at the same time ping a remote machine, I get ping responses like

64 octets from 134.169.34.18: icmp_seq=1 ttl=61 time=17360.4 ms
wrong data byte #0 should be 0x45 but was 0x3434 1c 84 3f c0 d3 1 0
8 9 a b c d e f 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
20 21 22 23 24 25 26 27
28 29 2a 2b 2c 2d 2e 2f
64 octets from 134.169.34.18: icmp_seq=2 ttl=61 time=16380.4 ms
wrong data byte #0 should be 0x45 but was 0x3535 1c 84 3f ce d3 1 0
8 9 a b c d e f 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
20 21 22 23 24 25 26 27
28 29 2a 2b 2c 2d 2e 2f

When there is no load on the PPP connection, the responses are normal.
This is wierd, as it appears to be saying that the ICMP packets are
garbled, yet the downloaded file (via FTP, i.e. TCP) are perfectly OK.

This has been going on now for years, and I am getting nowhere with it.
It's very frustrating, so any suggestions, hunches, anything remotely
relevant is highy welcome.

Cheers,
Mike Dowling

--
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understand any wrath therby incurred, but I'm no longer in any
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