5830 w/ 3 monitors, works for a few seconds

Hello,
I am trying to add a new ASUS 24" VW246H to my existing setup, which is:
Two Dell 20" 2007WP's on a Sapphire Radeon HD 5830 via DVI.

The two 20's having been working on that card for several years.
The ASUS is connected from its DVI port to the 5830 displayport via an Accel active adapter.

The setup works fine for a few seconds to a couple of minutes, then one of these scenarios happens:
1. The Windows desktop disappears and the keyboard does nothing, but they still all have signal and I can see the mouse hourglassing and I can move it.
2. They all go dark, and the driver flickers between displays - looks like the driver is changing modes.
3. One of the displays loses signal and the other 2 work fine.
I have to reboot to recover from 1 and 2. It comes back up on 1 or 2 displays.

I've tried using display groups in CCC, as well as just extending across the displays in either Windows or CCC.

The new monitor+adapter works fine by itself or with 1 other display, just not all 3 at once.
And the new monitor works fine on HDMI, but obviously I can only do 2 heads that way, not all 3.
I have the latest CCC running on Windows7/64.