thanks for the replies everyone.
I have already been through the preferences and can't seem to make any changes that correct either problem.
in more detail, the problems are as follows:
1. Photoshop will only use one core, which is VERY annoying when working with large processor heavy tools. I can literally see the computer choking as it refuses to use more than a single core and is starving for more cycles, which it should have access to
2. When using large files (particularly panoramic photo-merges), the activity monitor shows my ram slowly chugging down towards zero, and when it reaches zero, I get a warning that I am out of ram and the current process halts.
Now when using CS3 (I only started using CS5 about a week ago) this is the point that the computer would then start to use a hard drive for more memory (obviously processes would slow at this point!), but with CS5, it spells the end of any particular command.
So far I have tried almost everything, and in the end reinstalled photoshop. But still nothing has remedied the situation, and it is very concerning for me (and my work!). I have also tried setting five different hard drives as scratch drives, again to no avail.
Has anyone had any similar experiences? Any solutions?
Could this potentially be some problem with differences in .plist files between my boot and files drives? I am guessing that my next step would be a complete system reinstall?