Only time I ever saw that error was when I installed an updated graphics driver.  I ended up rolling back to the previous driver and it went away.  I believe what it is saying is that the share you submitted is not a valid hash which would indeed suggest that the hardware is making a computational error.  But more than likely (and your testing seems to bear it out) the hardware is fine and it is the way the miner is interfacing with the hardware, in other words the driver.

So what hardware and what driver are you using?  Your forum name is for nVidia hardware which you would normally want to use a CUDA miner with, but you are talking about an OpenCL miner which would normally be used for an AMD/ATI card.