I know it's been a while so you may have already figured the problem out, but...
Even though you have duplicates turned off I believe it will still replace an entry if two machines have the same DeviceID--this is sort of the opposite of a duplicate. Configure/turn on Inventory History for "Device ID" and "Device Name". If you check the Inventory History on one of the effected devices and you find multiple PC names for one device ID you must be using an image for your PC deployments (Ghost, Drive Image, etc.) and you have included the LANDesk client which is using the same device ID.
Also have you checked the Device ID of at least two of the PC's that are replacing each other? I bet they are the same. You need to delete all the device ID registry keys and files on the clients and also you really need to do the same in your deployment image(s), and if that's true you frankly you need to take the client out of the images for the future and deploy via ActiveDirectory.
You also have the option to deploy the agent post Image as a part of the imaging process.
The reg keys that must be deleted first if you are going to Bake the agent into your Images as as follows:
This will allow the newly imaged machines to get a Unique Device ID the first time the scanner runs.
Aye - the post to which rhyous is linking should sort you out.
And I completely share his stance on never. ever installing the LDMS agentry on an image. It's so simple to install it post OS-install whatever method you use, and you save yourself a whole variety of issues (all of which are detailed out in his post).
Please have a careful read of it, and take the information to heart :).
LANDesk EMEA Technical Lead.