Thanks Jon - my Mac has 8GB RAM, 1 TB HD (which is hardly used), an
AMD Radeon HD 6770M 512 MB graphics processor and is running OS X 10.8.2 with an Intel Core i5 processor.
The source clips are mostly mp4s saved off a Canon camera into a folder on my desktop, before being dragged & dropped into Premiere. They range in size between 7 and 166MB.
It also froze once when I'd created a title and dropped onto my timeline. I was adding keystones to change it's position so that the text scrolled in from the left and suddenly I got the spinning beach ball and had to force quit.
Most of the time that it happened though was when I was playing an mp4 in the source window, and would cut a piece of footage ('I' and 'O') and then drag and drop it onto my timeline. If I then went back to continue playing the source footage, that was when it was likely to not work, and I'd have to force quit the program and restart.
The spinning beach ball is an indication that your system is waiting for some tasks to be finished. In your case it could be that the CPU, which lacks hyper threading, memory, which is bare minimum, video card, which does not allow hardware acceleration and a single disk all contribute to this phenomenon. You are correct that the system exceeds Adobe minimum requirements, but those requirements are only for installing the program, not for using it and especially not with AVCHD or DSLR material. See some Practical System Requirements.