This isn't a problem necessarily with JDeveloper as such, but running JDeveloper makes this much more likely to happen because of the number of files it deals with. The "gam_server" which monitors file system changes can sometimes go, and I am sorry to use a technical term, "go mental" an eat up at nearly 100% of a CPU. (Luckily it is single threaded so it doesn't got to the dreaded 200%)
Turn out you can ask gam to be a little less aggressive by creating a .gaminrc file in your home directory. Mine looks like:
fsset nfs poll 10 fsset ext3 kernel
This tells gam_server to poll at 10 second intervals for nfs mounts but use the kernel for the main file system. This seems to help a lot on my machine as most of our source control system is via nfs mounts. I very rarely get the gam of death.
More information on this configuration file can be found here.