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memcpy --- EXC_BAD_ACCESS

Hi, can someone help me with this please?

I experienced occasional EXC_BAD_ACCESS with memcpy while i am reading back image data in DCLcallback.  
both my destination and source buffers are large enough for all the line copy operations as following. 
            for (cycle=0;cycle<PacketCnt;cycle++) //line by line to read the whole image
            {
                printf("\npacket copy: %d",cycle) ;                

                memmove(&pFrameDataWord[currentWordOffset],pPacketBuffer,4092);              
                pPacketBuffer += 4096; //removing 4 bytes header
                currentWordOffset+=4092/2;    
                printf("\ncycle:%d:\tPackCnt:%d",cycle,PacketCnt);
            }

the strange thing is that for the same memory allocation, no matter how many lines I read, it is always the last cycle of memcpy/memove that gets caught. 
If I read 5 lines, the 5th memcpy crashed, if I read 50 lines, the 50th has the problem, although the pPacketBuffer always contains 78 lines and the destination is set for 78 lines.

Debugger reported this:

Allocations will be placed on word (4 byte) boundaries.
- Small buffer overruns may not be noticed.
- Applications using AltiVec instructions may fail.
GuardMalloc-11
Allocations will be placed on word (4 byte) boundaries.
- Small buffer overruns may not be noticed.
- Applications using AltiVec instructions may fail.
GuardMalloc-11

Is this related and how do I avoid this? Thanks

Sufang
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