Well, yesterday I upgraded to 2.1.4, and this problem is back with a vengenace. 2.1.2 seemed fine, certainly much better than before, but now it's taking me 20 minutes to write an email. 1-2 times in each sentence the computer is pausing and not responding to input. 30 seconds or so later, it wakes up again, and whatever I typed starts to appear.
In fact, it's just recovered as I finished that paragraph, and the last 3 words I typed have just been entered in the email.
So the entire machine has been hung for the time taken for me to open the virtualbox website, find the forums, search for my post, and write an entire paragraph of text.
My host machine is seeing no performance problems. It's a quad core machine and the busiest core is only at 20% usage, the others are all around 5-10%. In terms of memory, I've got 2.3GB used out of 3GB, so there's still 700MB free, and it's only using 33MB of swapfile.
The virtual machine also seems fine - CPU usage there is steady at 1-2%, and it's using 588MB out of 768. Also, during these 'pauses', the task manager display is updating fine.
So it seems that this pausing is related to keyboard input somehow.