In what condition are you obtaining this plate/sheet?  Specifically, what heat treatments have been performed after hot rolling?  Has it been cold rolled?  There should be a temper designation after the 5052, such as "O" for annealed, or H14 or H32 to designate that it has been strain hardened.  If you are using a strain hardened temper, then the unwanted cracks are due to:

a) the elongated grain structure that develops during cold rolling, providing a preferred intergranular failure path

or possibly

b) inclusions that may be more prevalent/sizable in the rolling direction-- similar to manganese sulfide inclusions (stringers) in steel.

Options to consider would be:

1) Use the 5052 alloy in the annealed condition.  This will produce a more uniform, isotropic grain structure, that will be less susceptible to intergranular cracking during "with the grain" bending.

2) If you need a higher strength level than that produced by annealing, consider using a less strain hardened temper.  H32 temper is less strain hardened than H34, H36, and H38.  Less strain hardening means less elongated grains.

3) Investigate the metallurgical quality of the alloy with respect to grain size, grain uniformity, inclusion content, etc.  This may not be the easiest thing to do, as it will cost money to perform the testing, but your material supplier may be capable of doing this, and may already have some information on this.

4) Consider using a more formable alloy.  Aluminum alloy 3003 is a widely available grade in thin sheet, but I don't know for sure right now (I am at home, not at work) of it's availability in thicker plates/sheets.  Anyway, it is a lower strength, more formable grade, which may be suitable for your application.