The Treo 90 is in my collection as well as other models. But your post omitted any information about the PC in use. In short, this sounds like another PC and USB issue.
http://www.usbman.com notes many causes including out of date BIOS, old motherboard drivers or "other".
Bob
Can anyone help. I bought a used Treo90. It only hotsyncs when I uninstall and reinstall. I've gone to the Treo90 website and followed instructions to upgrade software, but still no hot sync. I've check the connections and hot sync mgr and all seems fine. I'm using the usb cable that came with the product.

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