Macbook Pro with External Monitor Keeps Readjustng
The displays's physical connection is disconnecting and reconnecting causing the Mac to automatically detect settings from the display upon re-connecting, this seems to indicate a physical hardware issue. 

Monitor Cable:

First  make sure the cable is securely attached to the monitor too.

Failing that, I would try a different cable and see if the problem persists, there may be a short or continuity issue in the wires within the cabling. 

Monitor:

Additionally try a different monitor and see if the problem persists, it is entirely possible that the Monitor's internal electronics may be failing.

The MacBook Pro:

If the problem persists after trying another cable or monitor it may be an issue with the DVI port or even the connection of the DVI port to the motherboard on the MacBook Pro. 

If you try gently applying very slight force to the cable end near the MacBook Pro while the monitor is connected in directions such as: up, down, left, right, etc and that causes a change in what your observing, the DVI port may be failing.