Data Abort after return from portRESTORE_CONT

hi all,
i am using AT91SAM7S256, IAR and FreeRTOS 7.0.
When program is during in portRESTORE_CONTEXT macro, sometime, the mode was changed to SYSTEM instead of Supervisor. and this time, when PC run SUBS PC, LR,#4 then program go in abort data exception. Thanks