Premiere Pro CS4.2.1 preview window has become jerky (more details)

Question

I am using a PC, Quadcore (Athlon II X4) 64 bit Windows 7, 4 internal Hard drives (all single partitions, NTFS), 8GB RAM. I am using Production Premium CS4. My graphics card is an ATI Radeon 4850 running the latest Catalyst drivers (or at least the ones from 3 days ago). I am using 2 TFT monitors, both running at a resolution of 1280x1024.
The hard drives are arranged as:
80GB windows drive (where CS4 is installed): 14GB free
500GB: Cache files stored here: 270GB free
2TB: raw footage for a new project: 844GB free
1.5TB: scratch files and all other project files stored on here: 204GB free
I put in a new Internal 2TB harddrive & installed a black magic plugin for some high def footage (the decklink software from www.blackmagic-design.com)
All preview playback in Premiere Pro has become jerky and unmanageable: only a few frames per second. Before installing the blackmagic plugin playback was smooth with all video sizes, now I can only achieve smooth playback with a tiny (~10% size) preview video.
I tried uninstalling the black magic plugin and changing catalyst control centre settings (regarding playback) neither of which worked. I tried to do a system restore on the windows drive, but this failed. I also tried disabling hard disk indexing for windows search in case this was having an effect, but it seems to have made no difference.
I have tried everything could find on message boards; old DV footage that was fine previously will no longer play smoothly it seems likely to me that this is a software configuration issue.
I have an ATI Radeon 5670 card which I'm going to try today. Other people have mentioned switching to nVidia cards, but I don't want to shell out for a new graphics card unless it is guaranteed to solve the problem once and for all.
Any help would be greatly appreciated, if any more details would be useful then let me know and I'll post more information.

Answer 1

I've just removed a large number of temporary files, uninstalled a few unnecessary programs (e.g. nero) and have moved the swap space away from the OS drive to being spread over the other drives. I've disabled disk indexing, removed unnecessary windows services and run CCleaner. I also followed (most of) your guide on setting a system up for optimum performance.
I should note that Windows Media Player doesn't have a problem with playing large files, and so it appears the problem is localised to Adobe software.
The time taken to render DV footage has increased dramatically almost tenfold. This is something of a guess, but could Adobe be using the CPU to render video instead of the GPU?
This might not be relevant, but the timeline bar flickers when I'm previewing a video.
Disk space is now:
80GB:    43GB free
500GB:   255GB free
2TB:       696GB free
1.5TB:    349GB free
I tried creating a project that didn't use the 1.5TB disk, but it exhibited the same problem.