My first thought is that there is a problem in your preview cache. You might try exiting LR and deleting the "* previews.lrdata" subfolder in your catalog folder.
I'm not sure where you read the you cannot use your existing serial number on a reinstall. That simply isn't true. Nevertheless, you could uninstall your standalone Lightroom 5 and then install it from the creative cloud application manager. It's a very simple transition. You remove the old version, and then install from the creative cloud application manager. The installation puts an icon on your desktop. You double-click the icon, Lightroom opens the same catalog you are using previously, work continues uninterrupted. I don't know if this is going to resolve the issue. But it's worth a try.
Thanks ManiacJoe and Jim,
I ended up dumping the cache and also optimizing the catalog and enlarging the cache size and restarted the computer and that has helped. All seems to be working today. I'm not sure where I read about not being able to use the stand alone version license key but I think it was somewhere on a Adobe site. But after all the reading yesterday I can't say for sure. I'm hoping all keeps working until LR6 comes out.
Appreciate your suggestions.