I know this might sound obvious, but are you (or is Windows) installing the correct driver?
I too had that on a Windows 98SE machine. It was working, but everytime I rebooted it found new hardware. Eventually, by taking it out of the slot and having a look at it, I discovered that Windows was installing a generic driver, when the hardware actually had an Acer Labs chip. Once I had chosen the correct driver from the supplied CD it worked fine.