Just few thoughts on my end, maybe someone more experienced, hardware wise, will give more info...
Both Program and Source panel have the option to reduce the playback quality, and this helps when playing big media - in the lower right corner, left from the timecode, there is a pop-up menu where you can choose from Full to 1/16 playback quality. This change affects only the playback, not the quality of the media. Also, you can right-click on the Program or Source panel and change Playback Resolution and Paused Resolution. I would stay the playback at the most convenient speed, and switch back an forth the Paused Resolution, as needed.
Don't delete the cache Premiere does, these are files that are actually speeding up the process.
Hope this helps.
Cheers for the reply. I did try lowering it to 1/2 res but, when I didn't notice any massive change I just assumed that it didn't make a differennce. Since reading your comment though I've gone down to 1/4 res which does seem a little bit healthier (for some reason though, everything below 1/4 is greyed out) but still - any improvement is a great help, so thanks for that.
Regarding the cache, I'm a little confused as to how this all works. In After Effects, when it gets full I have to clear it otherwise it can't render, I just assumed the same was true for Premiere. Wierdly enough, when I use the Premiere Preference to clear the cache, it still leaves about 20GB of files in the folder. I'll hold out on clearing it though if that's causing problems.
Thanks again for your help, much appreciated.
In After Effects you 'purge' the cache to free up RAM, in order to preview other compositions. In Premiere, it is a bit different, because this program works with different cache files and different ways of storing them. There are files used for previews/renders, created by the user - and files for caching information about media used in the project (metadata, waveforms etc.), created by Premiere. The only ones you can delete are the render files, found in Adobe Premiere Pro Preview Files (if you don't need them at one point; they can be easily recreated by rendering again). All the others (the ones you say that you delete through 'Preferences') need to be kept, because Premiere works with them. I assume this is the reason it takes that long when opening the project, because Premiere needs to re-cache everytime. Don't delete them and see how it goes.
Just as a reference point for you: I have a powerful computer (dual Xeon, dual nVidia, 64GB RAM), and I still have to decrease the playback quality sometimes to 1/2. The lowest I can go is 1/4, and I assume your Premiere allows decreasing at least to 1/8, with the rest of values grayed/unselectable.
I think you need a better graphic card first, one that is accepted by Premiere, for its CUDA capabilities. Check their website, I think your Radeon is not allowing the use of Mercury Playback.