I've had Oracle run away with my laptop like this once a couple of months ago, but I couldn't relate it to any particular action I was taking in OpenACS 4.5.
Nor could I replicate it later. It happened once then never again.
It really had the feel of being an Oracle problem.
Of course your problem could be very different. Do you have lots of users or content on your site? It may be that you hit a query that does an unqualified join on two large tables.
If your site's a development site with little content, though, as much as I hate to say it you may've run into an Oracle problem.
