Stale shares are shares for a block submitted after the next block is already being mined. Its result from network latency and fast block times and has nothing to do with memory errors or overclocking. Too much overclocking can definetlly cause memory errors and invalid/rejected shares though. Some of my Powercolor 4GB Red Dragon RX 570's run stable at 29.5-30.5 MH/s with a 1150 MHz core and 2075 MHz memory clock. I do have some that will only run at 1950-1975 MHz memory overclock at ~28.5 MH/s.