This worked! Cheers

However, I do believe my computer to have two issues. Did you have this problem in CoD4?
Because I did and the lagometer, as shown in the first post, pointed towards packet loss problems (green graph). Whilst the issue in the second game appeared on the blue graph.

Two different issues with same symptom - stutttering every 3-4 seconds. But it seems that this stuttering has stopped - I've done a lot of stuff with the computer aswell as changed network card.

Nah man, Never had that kind of problems.

In COD4, if you haven't changed it yet. /cl_maxpackets 100 in the console should greatly reduce your packet loss. I'm going to assume you have probably changed it, since it's a pretty commonly found tweak for the game, but worth posting anyways.