I don't know yet whether I should consider this good or bad news, but the fact is that I'm back online for now. Instead of replacing the splitters first, I decided to take the firewall out of the loop and that solved the problem. With a direct connect from the main box to the ADSL modem, there are zero connection problems, which probably means the firewall is at fault.
It could - I assume - also mean some cat 5 cable is faulty, disrupting the connection between the modem and the firewall, but it seems rather unlikely. I'll have to do some more testing to know for sure. It's a good day to stay inside anyway, with windspeeds up to 110 km/h predicted I don't feel like going out unless really necessary.
Notice : you may see FK disappear for short periods of time - if everything goes well that is - due to some maintenance being performed. If we return the latest version should be running under FastCGI resulting in an increased performance. I'm not sure whether that performance will be noticeable by end users/readers or mostly on the back end/server. Time will tell :)
Update : The new version seems to be running, but I can't get FastCGI working. Whenever I change the extensions to .fcgi and update my config file to point to them, they return a "file not found" error. If you have any clue on how to solve this - yes, the files do exist and the permissions are correct if you ask me - please let me know!