When I use my scanner I allways use select Twain source before trying to scan anything. Before that I would have a simular problem, the scanner would not allways be detected or it would hang while trying to scan and often ggive an error message. I'dhave to reboot before it would work again. Who knows that may be the correct way to do it for my scanner, I've never read the instructions so?
If you haven't done so try uninstalling the scanner drivers and all related software and start from scratch with a new installation of the whole package.
If the scanner came with a SCSI interface card I'd take that out of the computer if using the USB port.
Also downloading drivers patch for the motherboard may help with the usb issue especially if it's a VIA chipset which has a 4in1 driver patch that can be downloaded from thier site. With Intel's chipsets it's a little hard to find the correct drivers without all the correct information.
The first question should have been, did this problem just start happening or has the scanner done this from day one? If it's just started happening after working for a period of time it may be that the scanner is going south on you. Compared to the price of scanners and the amount of time I'm sure you've spent, it may be better to replace the scanner.
Good luck
