Memory problems of that type either come from the memory controller or RAM. Sometimes the BIOS will misread the SPD of the modules, so the response times are not set correctly. Too low DRAM voltage will also result in these errors without a memtest failure. I saw this all the time when I was playing WoW, as that is possibly the most memory transfer intensive game on the face of the planet. Sometimes even a light amount of data can be lost due to a capacitor being discharged before recycling. Memtest only tests immediate clusters for bad caps and transistors, it doesn't test whether or not it has enough long-term power.
For these scenarios, when Memtest86+ doesnt show results,
Prime95 is used to test the long term capability of memory. You have to run this program for several hours sometimes in order to get a confirmed failure. If errors pop up ever (more than maybe one error in an hour), your ram is not set up properly.
To fix this, go into the BIOS and increase the SDRAM voltage by 0.1v only. Most memory modules can run at ~2.6v but can handle up to ~2.8v with proper timing. Also check the timing ratings in your system BIOS and compare them to the timings as listed by the MFR. If they don't match, then fix it. If they do, increase CAS latency by half a cycle. Also increase the larger number (I forget what its called) by one cycle.
Anyway, try that out and see if it works.
EDIT: And yes, your memory controller might still be bad. If you have an A64, you must replace the CPU. Otherwise, the motherboard will need to be tweaked, and possibly replaced.
