A further observation on this:
Recently an (ex-) ISP has had problems and we were getting very high packet loss intermittently at one site. The packet loss exceeded 70% at its worst but built up gradually over typically 10 minutes or so. The reason that they are now an ex-supplier is that over five days they refused to accept that there was a problem with the wholesale connection and it has now been rectified, I suspect, through the efforts of another customer.
Anyhow, as the error rate increased, first the OSPF (MD5) over PPTP (MPPE 128) link went down although PPTP stayed up. Then PPTP went down and, ultimately, the router lost ip connectivity to the internet. So it seems that OSPF is VERY much more sensitive to packet loss or related errors that the bearer PPTP link.
Sadly I do not have logs to be able to put a threshold on this observation which is entirely empirical.