580 EX reverts to "M" from "ETTL"

I'm having a problem with my 5D bodies, STE-2 and 580 EX. I've referred to the manual, but can't find the solution to this. I routinely use this system as follows:
5D set to "manual" and I adjust exposure to suit ambient
580 set to "slave"
STE-2 set to "i" (first "on" position)
The problem is that ever since fooling around with manual flash settings one day ("M" mode) I can't get the flash to retain the ETTL setting. I can reset it to ETTL, but then after some period of time (it seems to be random) the flash will revert to "M".

I must be doing something wrong, but can't figure out what. I'm getting quite accustomed to looking at the flash and resetting it myself, but that isn't too darned efficient.

So how can I set it to "ETTL" and get it to stay that way?

Michael, I'm guessing you have not installed the very newest firmware for the 5D. This was a problem that occurred (only when using the STE2 BTW) and was corrected with the new firmware--sometime this summer. It drove me crazy until I wrote Chuck Westfall about it (and it was discussed a bit on other forums last year). Just go to the Canon site and get the new firmware, install---and the problem goes away. I did it in Sept. knowing I would be using the STE2/4 flashes for a job in October. Up until then I didn't bother since I don't normally use flash. My understanding, though, is even though it 'reverts' to manual---it really does work as ETTL. But who knows about that. Since the firmware upgrade corrects it, that's an easy solution.

Diane

Thanks, Diane. You're right. I did not upgrade firmware on either body since there was some issue with serial numbers and mine fell into the group that had to be sent back to Canon. Can I now upgrade firmware to latest without sending the bodies in?

Rats. I guess I'm getting feeble-minded. Both bodies have 1.1.0 firmware and that's the latest, I think. There's no question that the flash changes from ETTL to Manual so I guess I'm stuck again.