Wonderware Tag Update Issue

I have a system with a strange problem. It is a Simens 505 controller programmed with APT 1.9. It has a CTI2572A ethernet card that talks to two independent Wonderware stations each with their own CTI DDE Server. We have ONE tag out of approximately 2900 that has an EXTREMELY slow update time (on the order of minutes). If we go online with the controller in APT, the value updates normally, following the analog signal in real time. If we drive the signal to a new value and shut down one of the DDE servers, we get a one shot instantaneous update to the current value on the running system, but then it goes back to taking minutes to show new values.

Anybody have any ideas?