I have had issues with bluetooth turning off randomly since the beginning. I tried all suggestion on this board including registry changes and cab installs, as well as changing bluetooth headsets, etc.
So, I started experimenting: I turned bluetooth on, then went into the options and made sure the square was checked for bluetooth, went back out to main screen, then went back into the the option screen. Next I used a switch program to go back to desktop leaving the bluetooth option screen open in the background. It has now been 4 full solid days since my bluetooth has turned off even once!
Try this and please let me know if this works for you.

I'm all for finding an alternate solution, but I don't understand how you're keeping the Bluetooth option screen open in the background. I use a switch type program too, but when I go to another screen, it appears to close the Bluetooth option screen. Even if I didn't have a "switch" program, wouldn't navigating away from the Bluetooth option screen be accomplished by clicking Start and then Today (instead of clicking the OK button)? In either case, when I check Running Programs the Bluetooth option screen doesn't appear to remain open in the background. Am I missing something?

The only time my BT shuts off automatically is when I use a Plantronics Discovery 665 at the same time as a Tomtom Navigator 5 and BT GPS receiver. Even still, it works most of the time and only shuts off intermittently, therefore, I can't correlate it to any one event or configuration.

It is now day 5 and my bluetooth hasn't turned off once. So, it is working for me. The option page wouldn't show on the list of running programs, however it seems to stay open in the background since I have made no changes to the phone, added any programs, made any registry changes except for the aboves steps. If you are having issues with bluetooth turning off, give this a try: no harm done!
And please post if it helps.

I am using a custom ROM from AKU 3.5 and I have never had the bluetooth turn off on me.

Currently my wife and I are on AKU 3.5. I personally have never had the Bluetooth turn off on me regardless of ROM version, but my wife experiences it frequently with AKU 3.5 (as she did on prior ROM's). I was really hoping that the ROM upgrades would have resolved the problem, but no such luck. I have our 6700's configured identically, so it must be a hardware issue with her phone. I didn't want to get a refurb replacement phone, so I was looking for something that might resolve the problem without going that route.