This may not be related, and I don't know much about USB current, but would the very old, original USB computer ports actually not be able to provide as much electrical current as the newer USB ports ?
Yes, v1John, this can definitely be a problem. The older 1.0 and 1.1 versions of USB almost never supply the current needed to make a pt work properly. Unfortunately, while the newer systems that have integrated USB 2.0 (and the new USB 3 standard which is just starting to trickle into the market) should provide ample power, there is no guarantee that they will.
Some setups have lower-than-spec power outputs and depending on the setup sometimes having other
devices plugged into a system can lower the total available output which can cause problems as well. I tend not to plug my pt into my computer because I'm concerned about potential damage.
I use a usb / ac wall adapter or an ac powered USB 2.0 hub that is not plugged into my computer and get along well. Many folks don't have any problems when they attach a pt directly to their computers, but I'm not comfortable enough to do it myself.