Chris G wrote:
One noticeable thing apart from the load average being so high is that my 8Gb of memory gets eaten up too, but there seem to be no particular processes using either lots of CPU or lots of memory.
This may be simply down to the kernel using unallocated memory for disk cache etc, cat /proc/meminfo would probably tell you more info in this respect and you will most likely see that the vast proportion of memory is "cached" which means it is being used as a filesystem cache as there is nothing else for it to do.
top only uses the first two values so from there it looks like you are almost out of memory.