We have a similar problem, except that it is not only for IMS content packages that resources do not want to open. Even .doc and .pdf files keep on churning in IE ad infinitum. Clicking on the Refresh / Reload or pressing F5 (most users use IE) seems to solve the problem, and once it has opened a resource, it seems to load it a next time without refreshing. Firefox doesn't seem to present the same problem, but Mozilla does. I suspect it started after we upgraded to Moodle 1.6.2 (2006050520).
Thanks in advance,