Lpc2000

I have project to use LPC2214 ,
but I found sometime i will got Data abort.
I use F10 to run the function call is ok ,
but if i use F8 to step the function,
i will got Data Abort when run return instruction,
I found the mechine code is ldmfd r13!,[r4,r10,sp],
and R13 is not correct , when occur data abort (it seems just shift 4 
bits),

anyone have this kind of experiance ?