General :: Memory Cache Losing Sync With Disk?

(Ubuntu Linux server, 64-bits)I was troubleshooting a problem with a file (~3.0 GB) which I had just downloaded, but it was failing the integrity test, when I discovered something really unusual.First this is the MD5 of the file after download, which didn't match the expected value:

~% md5sum media.iso
5d74facb904cc1765a468354908a8f34 media.iso

Some time passes, nothing should have changed the file during this time, but when I went to check the file again:

~% md5sum media.iso
a5b97c5016afb39bd67ccfc3fa6ca59e media.iso

This was really unexpected. Since I have a lot of RAM, I suspected this was the effect of caching and something was going awry with it. I decided to retry with the whole file from disk, for my surprise:

~% sudo sysctl -w vm.drop_caches=3 # This linux command invalidates
vm.drop_caches = 3 # everything in the memory cache.
~% md5sum media.iso
2992aa6270f6e1de9154730ed3beedc1 media.iso

I redid it and now it seems to stay consistent, although this still isn't the value I was expecting. Certainly, the contents in memory cache were different from the contents on disk.This is the big problem.To fix the download, I created a torrent on the source machine and opened it in the target machine. Five 1MB chunks out of ~3.0GB failed integrity check. I used the torrent to fix these file chunks and how the file integrity is ok.The problem now is to determine where the data got out-of-sync.

I tested the memory with memtest86+, all but the bit fading test. I was expecting to see some failing memory module, but there wasn't anything. Everything is ok.Filesystem is Ext4, over LVM2, over a 3-disk RAID5 array.Ext4 is considered stable, and if data were inconsistent between disks, mdadm would have warned. But there is nothing in the logs. S.M.A.R.T. error logs are clean, the disks are new (have less than 30 days of "power-on-hours").I'm looking for information about any data-loss bugs in my current kernel (2.6.35), but there doesn't seem to be anything, as far as I looked.what else I could check, or where exactly could be the defect/bug?It is a Ubuntu 10.10 64-bit, Core i7 930, 6 GB non-ECC RAM.

Update: I confirmed that the files are being correctly written to the disk, the pages are being altered after they are read from disk, while in memory. I did a lot more memtests (I left it doing bit fade test overnight),and still nothing. All memory modules seem ok.Some more tests:

~% md5sum media.iso
cc8bcf1ce67ff7704eadc2222650c087 media.iso
~% cp media.iso tmp[code]....(direcat is a version of cat that reads with O_DIRECT, that is, bypassing page cache)There is a clear pattern: it always happens to the 2nd byte in a 16-byte alignment. In that byte, almost always the bit 4 (LSB) flips to one, but there was one instance where bit 2 flipped to zero.