No sign of 52317!
It could have been something (file sharing software maybe) that was running at one point and the router has just kept the forwarding rule in place even though it's not currently in use.
The router is very old - prolly circa 2003 - but I have previously swapped it for a rather nasty Thompson thing we got with our last broadband package, and we still had problems. I'd be prepared to go and buy a Draytek or something if I thought it would solve the problem.
Ten years is a very long lifetime for an ADSL router, it could definitely be on the way out - as far as I can remember pretty much every Thompson branded router I've dealt ever with has been very nasty indeed so not sure your swapping it proves anything! Draytek are almost always very stable (certainly if the firmware is up to date), though I quite frequently find that they are a bit over-conservative when it comes to ADSL sync speeds. Quite a lot more expensive than the run of the mill rubbish though...
I should also ask... on which devices have you noticed the problems most, and are they all wireless? Also I've seen 3rd party firewalls cause some very, very strange networking issues on Windows, are you using the standard Windows firewall?