GS748TPS Stacking Errors

I have 4 GS748TPS's and 1 GS748TS in a single stack, however two of the GS748TPS's seem to cause problems when they are in the stack. The whole stack will continually reboot and i'm seeing the following messages in the Flash Logs on these units

Emergency %SYSLOG-F-CORRUPTEDCONTAINER: Fatal error ***** FATAL ERROR *****

Reporting Task: EVGN. Software Version: V5.2.0.5 (date 17-May-2012 time 10:55:34) 0x144c60 0x1421fc 0x52a69c 0x358de8 0x70e628

0x719860 0x725f90 0x69f72c ***** END OF FATAL ERROR *****

Alert %SYSLOG-A-NONPRINTABLE: Formatted message for ComponentID 1 application 0 (Box

Application), message 10 (DISPATCHER-SEND-FAILED) contains non-printable characters message string is : $$$%Box-F-DISPATCHER-

SEND-FAILED: Function BOXP_send_dispatcher_event: failed sending dispatcher event 24059636

All the units seem to work perfectly when they are not stacked, am i doing something wrong here or are the units broken and need replacing? I can't find any references to messages like this anywhere on the net so i'm presuming that this isn't a common occurrence.