Does Bitcoin-QT use more CPU when its idle than during synchronization? I am running the standard client on a micro Amazon EC2 instance. When I am synchronising Bitcoin with the network, it uses about 50% of the available CPU. After it is fully synchronised, I leave it alone and it cranks up to 100% by itself.

What can be causing the standard client to behave in this manner? Is it due to possibly high transaction volume, sharing information with peers, or perhaps something else?
No, according to the information on small and micro instances at this link.
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/concepts_micro_instances.html
Bitcoin.conf ( the Bitcoin Configuration file ) may have an option set to 1 to activate mining within the client software.  If you have it set as ' gen=1 ' it will try and ' generate ' coins ( mine on it's own ) and that drives CPU usage up.  Also, according to the graphs on that link, it should not ' jump ' to 100 percent use, each instance should drop down to a low usage level for small and micro Amazon EC2 instances.  There may be other programs running on your Amazon EC2 instance that can also drive up CPU usage so I recommend checking your files starting with looking for any instance of Minerd ( cpu miner ) and removing it.