As I said above

The display port to DVI adaptor only supports SINGLE LINK resolutions - so probably only 3x 800x600 - not 3x 1280x1024 if you are using a standard TH2D-Digital edition.

Higher resolutions are only available if you use the DISPLAY PORT edition connected to the DISPLAY PORT on your graphics card.

When working with VGA - there are sync issues on output 2 of the TH2G due to lower sync levels. Depending on your kit, this sometime create 'messed up' images. The way round this is to use a VGA DA that correctly buffers the signal before it gets to your projector / screen. (Kramer VGA DA's seem to work well) This might be what you are seeing?

The difference between your Macs is the graphics card.... not all graphics cards support all these TH2G resolutions.

DisplayPort and Mini-Diaplay-Port will only support SINGLE LINK resolutions natively with passive adaptors - so when connecting a TH2G, you wont get the same FULL resolutions that you do when you connect TH2G to a DVI-D output from the graphics card (which supports DUAL LINK - hi res)

If you want to use a DP or MDP output from the graphics card - you have to use a TH2G-'Display Port' edition to get access to the full resolutions.

To give an example:
MacPro 5,1 with Quadro8000 (DVI-D + DisplayPort)
1x TH2G-Digital
1x TH2G-Display Port

I can get:
6x 1280x720@60hz
6x 1280x1024@60hz
4x 1920x1080@60hz
as well as other lower resolutions

hope this helps

S