The resolution is different (NTSC= 720x480, PAL = 720x576), but they're both 4:3 aspect ratios (non-square pixels). PAL has more lines of resolution. This might be the difference you're noticing.

But when you watch Doctor Who in NTSC, it's not 25fps converted down to 24. Instead, a type of pulldown is used (similar to the 3:2 pulldown used to get 24fps material to NTSC's 29.97) in order to "add" 5 frames without actually adding any frames. So you don't have the 4% speed difference that PAL viewers get when watching stuff shot at 24fps (they have to bump it up to 25fps, which makes it run 4% faster).

So you might just be seeing some of the slight image judder that comes with using a pulldown method and repeating fields and all that. Watching it on a computer monitor at the proper 25fps (i.e. download a torrent, or get a region 2 DVD) might help.

But since Planet of the Dead (so for the last 3 specials, and all of series 5 and 6), the show has been filmed at 1920x1080 high def, 25fps. So the resolution is the same at US HD (1920x1080), but the frame rate is again different. So you have the same pulldown business going on.

Honestly, I think you're just seeing the difference in cinematography styles between Series 1-4 and Series 5-6. The newer series are shot with more movie-style lighting and such, whereas the earlier series wear their "video"-ness on their sleeves a bit more.