After 8 months of working fine with 4 sticks of 4g Kingston HyperX Genesis in my Asus Z87-K it's now decided that it doesn't like them!
Each individual stick is fine in each individual slot as is any combination in pairs or even threes.
Put the fourth stick in and thousands of errors wthin 2 minutes of memtest running!
 
I got within abot ten minutes of completing a project last night before this started with multiple freezes of Sonar and multiple BSODs.
 
Heeeelp!! :(

It's decided that it doesn't like 3 sticks now!
 
Ran OK for most of the day on 2 sticks of RAM so I managed to finish the project and start the next one.
 
A bit annoying to be back down to 8Gb again!

Anyone?

If it was working fine before, and now it's on the fritz, then something has gone bad.

So have you isolated each stick of ram? IOW, make sure that stick A is good, and stick B is good and so on. Mark each one with a letter or a number. Then try each one by itself (only one stick at a time) in single channel mode running memtest and see what happens. If you find all are good, try them in dual channel mode and see what ya get.

If all turn out bad maybe it's time to look at MB. I would find it hard to believe they are all bad.

Then let us know what goes down.

Memtest, by its nature is not only testing the memory, it is testing the circuits that support and control the memory. I understand you to say that the individual sticks test OK when inserted by themselves but you get errors and unspecified blue screens when they are all installed or when three are installed at once. The fact that individual sticks pass, tends to indicate that it is not a problem with the memory modules themselves. One possibility is that there is a problem delivering sufficient voltage to the memory. The most common reason for that to occur after a period without a problem would be corrosion on the socket or a bad solder joint which would increase the resistance and decrease the voltage. It could be that the full load of memory puts more drain on the circuit causing a voltage drop, that is not found with a single module. First you should probably inspect and clean the memory sockets and the contacts on your modules, as well as the connections of the power supply to the motherboard, although a power supply problem is unlikely if the machine posts at all. You can check the nominal voltage supplied to the memory in the BIOS, and if it is below what the memory requires, probably boost it to the specified voltage there as well. Unfortunately, about the only practical way to handle a problem with the motherboard itself is to replace the whole board.