Thank you all for the useful input. The memory that was bought 4 months ago had failed despite passing the Windows Memory Test. Lifetime guarantees mean nothing here in Bali. I have a stack of failed memories and video cards. Other than refitting the good memory I refitted the video card after putting new heat sink compound on the chip as I was concerned about temperatures. Its running at 52C at present, which is OK as we are generally at 26 to 28C at night. The computer has run for 10 hours so it looks like the problem is gone.