Could this be the mapsize bug? When you play a game on a custom map, it uses the settings (corruption, OCN, tech costs, anything that depends on map size) from the last random map you played. The way to fix this is to start a game on a huge random map, then go back to the title screen and start your game on your huge custom map. If you had played a smaller random game before your huge world map, it whould be using the OCN from the smaller map.
Shane: Pentium means OCN, not OCP. OCN = Optimal City Number, it's different for different map sizes, and affects rank corruption.