If the computer came with its original IDE hard drive, that disk could be at the end of its useful life and causing the errors you're seeing. Booting the Mac off an install CD and running Disk Utility may catch and repair errors, but for a hard drive that old, I'd strongly recommend replacing it with a newer solid state drive which will be more reliable and also faster.
The G3 B&W can use a modern 2.5" SATA SSD, but will require an IDE-to-SATA adapter like [link|https://www.amazon.com/StarTech-com-Drive-Optical-Adapter-Converter/dp/B00EOJNGC2|this|new_window=true]. Also note that this machine is limited to drives 128GB or smaller.