Xerox Phaser 7400 - bands on transfer belt

[edwardsoffice]

Hi All,
i'm hoping someone may be able to help me with aproblem i have.
We have a Xerox Phaser 7400 that has been ok, BUT expensive, but has developed an error.
When printing the backs of the copies are completely black, this is due to the transfer belt retaining the toner in Bands.
Hence each colour is represented in a band about 3 inches wide.
If you clean it, it comes back. We have changed the transfer belt but it hasnt done a thing, it looks like an overcharging problem.
I am just looking for any sugestions on where i should start.

Thanks

M