From the way I see it there is something in that SIMBA <-Orange-> Roaming Network link that is giving problems and to me it sometimes looks like the issue is packet delays that are giving weird results on the mobile phone...
For me in Germany Roaming normally works but then suddenly the roaming network is gone and the mobile (and not only my one) is unable to connect again to the roaming network. It will not automatically reconnect. I normally have to push it, by changing the "Network select" to manual and select one of the allowed roaming networks and this will fail, then I try another one, that fails, then I give it a minute or so, change back to automatically select the network and it will eventually be connected again... This most of the time happens when I am moving around, so I presume it happens at the handshake to another base station. This is where my theory of packet delays come in... And yes, I had the phenomena that you had about being "downgraded" to EDGE. It is either Offline or EDGE... I reported this a few months back to SIMBA but honestly getting this issue to the right person within SIMBA is where the complication is. You need a collaboration of SIMBA, their "Roaming Proxy Partner" Orange and the actual used roaming network to pinpoint the issue down. Funny enough, I never had these problems in "Orange" home turf France... It works a treat there, never had any issues in France... Another hint for a packet runtime or delay issue, because in France the relationship is like a direct roaming agreement between Simba and Orange.
I hope somebody from SIMBA is reading here that knows how to tackle that issue and can have a look into it. The issue is not consistent and there are good weeks and bad ones, once again coming back to my theory of handshake delays between all the partner networks involved. I never seen this happening on a straight SIMBA roaming network, only happens on "Orange brokered Roaming agreements". So... it must be the middle man who is the problem ;-) ...
You can try to manually select a non authorised network, let your mobile try until it says Not able to connect or whatever and then change it back to automatically, it may work... But maybe Greece is different, never roamed there with Simba.
Good luck, hope it will work out!