When I try to resize a partition I get: "Partition failed with error: MediaKit reports partition (map) too small."

Hi, I'm using an iMac G4 with Leopard and a 500GB hard drive that I bought off of ebay. The computer came with Leopard, but it was sluggish and laggy and I couldn't use spotlight without it lagging. I wanted to go back to Tiger so I bought a retail Tiger DVD from a computer shop. I still wanted to keep Leopard because some my application that I still have to use for PPC require 10.5. I went to Disk Utility on Leopard to partition the drive in half, I selected what sizes I wanted them to be and hit Apply. I went and got the mail, erased and replaced an old cable box, and other stuff while it was partitioning. When I came back everything was ok, two partitions were created except the Tiger one was smaller than it should be. There was empty space below the partition so I pulled the expander on the Tiger partition and made it bigger, then I clicked Apply again. When I clicked apply a error came up saying "Partition failed with error: MediaKit reports partition (map) too small.". I restarted into the Tiger DVD and I opened up Disk Utility and repaired the disk, it fixed two errors. Booted back into the Leopard installation and tried it again, same error. I then booted into Single User Mode and ran "/sbin/fsck -fy" it fixed a couple errors and then I typed "exit". It booted back into Leopard and I tried to resize it again, same error. If you have any idea whats going on, I would like to know. Usually I'm very good at fixing computer problems but I'm really stuck here.

-JP