Help with a cavity and patch simulation with CST， please

Hi everybody,
I am trying to simulate a patch fed by a cavity with a slot on it (find atached the picture).The resonant frecuency is 9.6 GHz.I am using CST Transient solver since I also need the farfields. The result I am getting for the S parameters is also attached.
It seems like there are some kind of instability but I don′t know how to solve it.
I first simulated the patch fed by a slot and a microstrip and I got satistactory S11 parameter.I also simulated the cavity with the slot and the 50Ω coaxial and it also worked.It′s when I try to put the two pieces together that I get problems.I tried to move the excitation probe to see if it was a problem of mismatching but I always get the same kind of response.
Can anyone give me any clues?I am using CST 4.0 since it′s all I got at the moment.That's why I don′t upload the mod file.
Any suggestion will be welcome!
Thanks in advance.

Hi,
I have this problem too, sometimes. I do not know why, maybe dielectricum is problem. Try increase mesh cells. Or you can use Frequency Domain solver, it will be better.

Thanks for your reply mauloftin.
I had one mistake defining the boundary conditions (I had not define open boundaries).I increased the meshing and it seems that it has now started to work!:D
Now I only have to optimize it but it will certainly take some time because of the meshing refinement.