HP4si duplex shifted output problem

[Art Hughes]

Hi,
I have an HP4si printer with duplexer option.  I'm using mandrake 8.1
with CUPS using the "Foomatic + ljet4" driver supplied via GS-6.51. 
When I print a file in duplex mode 

"lp -o sides=two-sided-long-edge"

the second or even page image printed on the back side of the paper is
allways shifted to the right by 0.75 inch.  If I print to the same
printer from an M$ NT4 machine with HP's driver the printer prints
without any problems.

Any Ideas?
Thanks,
Art