CPW simulation in ADS using momentum and co-sim

Hi all, i found some discrepancies when simulating a CPW in ADS momentum compared to when co-simulating in ADS the same layout. Wonders if someone can help clarify things. It's a bit involved so I thank you so very much for your patience.

Situation:

We are simulating a coplanar waveguide (CPW)using momentum and
co-simulation. A CPW calculator
http://www.microwaves101.com/encyclopedia/calcpw.cfm gives the dimensions
for a 50 Ohm CPW to be:

ground plane to signal track spacing=0.12mm
track width=1 mm
for
dielectric constant=4.5
and track height of=1 mm

I've created a layout in momentum that incorporates these dimensions in the
attached project file.

Problem:
Using momentum (pic: CPW50) and using ground
reference ports (P3,4,5,6) and internal ports (P1,2, i need to use internal ports since a similar design will be incorporated into a more complex system where
components will be attached to the ports),

we got a flat, broadband S21 (pic:Mom_flat_S21). and if we keep this config for co-sims, we get a flat S21 and good eye diagram.

But when the GND reference ports are removed and substituted with internal
ports (all P1-P6 are internal ports)

and a co-simulation is done, (pic:coSIM_S21),
a non flat S21 is obtained, (pic:coSIM_S21_non_flat)

when an eye diagram simulation is done, (Pic:coSIM_eye), a closed
eye is obtained even though the impedances are matched,
(pic:coSIM_eye_result)

Request:
Can you please advice:

1. How to get a flat S21 in co-simulation mode like in the momentum only
mode using all internal ports?

2. Get an opened eye diagram in co-simulation with all the impedances
matched at 50 Ohm when all ports are internal ports?

Or is it not possible?

Thanks for your time and looking forward for your response
raebrm