As usual - I can't stand things/hardware that does not work at 100%. Hence - I ordered a replacement board, and put it in place today. The new router board is the next generation, means that it has Gbit ethernet ports, and provides a USB Port as a Micro SD port that could be used. A test has shown 38MiBytes of FTP transfer speed - which is a good indication for GB Network.

It took me longer to actually get the new setup running - as the ethernet ports have been rotated somehow - making the backup of the old setup not compatible.
After reconfiguring the interfaces and the firewall rules, everything is running smooth now on the router side. As som OID's have changed for the internal Monitoring, I also had to adapt these on the MRTG Configuration.

[Update]
After having in place the new router, the connection still dropped from time to time. A new call to my provider showed that the modem (Turbolink 1203 ADSL2+ Annex B) on their side is Ok. I then decided to connect a HUB (yes - old sweet HUB, and I'll never hand it out) and saw quite some errors on that short link. A short data packet capture showed that there were many CRC errors on the link. I then decided to buy a new ADSL2+ Modem. The Netgear DM111PBL-100GRS was the one I took (Have made very good experience with the Netgear Type products, especially on ADSL link). Hoked it into the data flow, and all issues were gone.
I however wanted to know if the old router RB493AH was broken, or if it only was the old modem ? Hocked the old one in, and checked the link on the PoE/Ether1 Port. Still the issues. Note - the problems are existing on both sides. The old Modem, and the old Router. hence - both are broken. Probably due to a power-surge on the phone line (can't be the normal power - everything is secured through a USV with power surge protection). However - I can't protect the phone line itself. it would drop the the max. ADSL Speed to ADSL v1 -> 6MBit downstream.