23LC1024 Hang Up

Hello everyone !
I use 23LC1024 , but sometime it hung up (cannot read or cannot write) and do not resume without power down .
Could you teach me what happen ?
 
I use it for communication between PIC24FJ32GB002 and the 8051 embed processor .
They share 4-wire SPI-RAM bus and have 2-wire handshake-line for arbitration .
Each processor first get permission by using handshake-line then start drive SPI bus .
RAM contents are 64k-byte FIFO buffer and 2*2-byte read/write pointer . 
Almost it work well more than 30 minutes .
 
But sometime it hang up , and read/write pointer read 0x0000 (or 0xFFFF or some constant value) .
Also writing may be ignored (it cannot check by reading) .
I try to RSTIO command if it occurred but the phenomenon is not changed .
Restart each processor by ICE , but not changed .
Only recover way is power down .
 
Best regards , thanks .