![Icon Biggrin :icon_biggrin:](./images/smilies/icon_biggrin.gif)
I bought the American version of the Complete Series (the one with all the extras) for Christmas. I have plans to watch the extras with him sometime, so when we do that, I'll pop season 5 in just to take a look and see if I notice what you're talking about.
I have all 7 (6.5) seasons on DVD in my own collection, but these were the individual season releases. Season 5 looks absolutely fine, so I concur that whatever you're noticing is not a problem with the original DVD releases (or transfers).
Of the individually packaged seasons, I think season 1 is the weakest in the transfer. More film grain, more noise in low light, more compression artifacts. Not sure how much of that was due to poorer mastering to DVD or how much to native aspects of the film as it was exposed. While I think the show always had a high production budget, it's probably true that they were able to afford a few more lights after the first season.:-)