2d array slicing problem

[jeg]

dear all,

i'm an astronomer working with 2d images -- 2d numarrays. i have a
script which basically does some operations on some images, and one of
the first steps is to find a galaxy on an image (at, say, a known x,y
coord), and create a sub-image by slicing out part of the larger array
to create a more managable smaller one (e.g. 50x50 pixels from a
1000x1000 array).

i was running this fine under python 2.3.4, but due to the slowness of
my machine, moved to a speedy 64-bit linux box running version 2.4.2.
arrays were no longer being sliced in the correct way, for example:

B = A[3:7,6:10]

generates different Bs if i run an identical script on the same image
on the different machines.

note: we deal with FITS format images, which are read into numarrays
using the pyfits.py module. does anyone know any obvious reason this
should be happening? 

many thanks if you can help, 

jim