I had my previous motherboard ASUS M4A785TD-M EVO exchanged (guarantee), as two condensators had dried out and leaked. Originally Windows 7 64-bit claimed the freezing issues I had back then to be caused by faulty memory, but naturally I did not believe it. Especially as there was a clear fault on the mobo.
But. As a replacement I got ASUS M5A78L-M/USB3 -mobo. I installed the memory modules just as they were on my old mobo. Channel A had A-DATA AX3U1600GB2G9 DDR3 1600 Mhz 2 GB x 2 pcs and Channel B had A-DATA AD31333E002G DDR3 1333 Mhz 2 GB x 2 pcs.
Same freezing continued as previously. So I decided to pursue the issue of a memory fault as now I was sure that the mobo is not at fault. I removed the 1333 Mhz modules from Channel B. And then I realised that the colouring of the slots was different from the M4. Now A1 and B1 and blue and A2 and B2 black. M4 had black Channel A (A1 and A2) and blue Channel B (B1 and B2).
M4 manual did not give any instructions how to set the modules, so I followed the colour coding. M5 manual instructs to put the modules to blue slots for "better overclocking performance". So I followed this, now only the two 1600 Mhz modules are in blue slots A1 ja B1. Works perfectly (so far).
So what I really would like to know, is that if you have two pairs of different make of memory, do these need to be in (A1 and A2) and (B1 and B2) or (A1 and B1) and (A2 and B2)?
I have not found the definitive answer.
(Only that why in earth did you buy two different pairs of memory. Well back then in 2009 memory was not that cheap, so I had to take what I got).
Both mobos indentify the modules correctly and run all with 667 Mhz / 3:10 9-9-9-24-33-2T (Cpu-Z and Sandra). One question remains, why did this combination work well for over two years before the problems appeared. And of course the 1333 Mhz pair might still be faulty, I will update you on that once I have the energy to continue testing.
I will reply to myself as I got some good advice on another forum that might be helpful also to others.
So the problem was that I had four DIMM´s that all were all double-sided (DR). The Phenom memory controller is just not able to cope with this large amount of memory.
I ended up selling both 2 x 2 GB kits away and for the same money got a 2 x 4 GB kit of Kingston HyperX Blu memory (KHX1600C9D3B1K2/8GX).
So now I have 8 GB of memory again and no problems.
I have learned now that AMD Athlon and Phenom processors the optimal configuration is one DR DIMM per channel (or two SR DIMM´s).
I wish AMD and ASUS manuals would have mentioned this at least in the troubleshooting section.
Ivy Bridge and MSI next time!