|"I have installed windows 98"|
I would re-install Windows 98 without 1GB RAM for starters. While I have successfully run it under 1GB, I have always installed with 512MB or less to avoid instability during installation. It may not have installed properly with that much RAM.
iMac G5 1.8GHz "Hero" PowerPC
2 x 1GB DDR PC3200 SDRAM
160GB WDC WD1600JD-40GBB2
Mac OS X Leopard Version 10.5.8