Of the 3 full drivers (PS,PCL6,GPD-PS and GPD-PCL6) on Xerox.com, which have you tried?
They can't all have the same issue unless the software is the problem, the drivers are made by different groups and are totally independent
Maybe a little more information. Any batch of checks larger than 6 will stop printing the signature on the check after 6 pages. The signature is an embedded tiff image. Printer is configured with maximum memory. The software that is being used to generate the checks is Exstream Dialogue.