Hi I'm new to this forum and I just read your post.
I have been in trouble with my previous selespeed also.
I fear you installed the sensors in the wrong way. When you take them of they turn, because there is a spring inside. So try to install them as closely to the original position an turn them into position then so you can secure them.
On my 147 2.0, they only turned a little from the installed position, so it shouldn't be neccesary to turn them half way round.
The symptoms you previously had are most likely due to bad electrical resistance in the sensors. Measure the resistance between the pin A and B this should be a maximum of 1200 ohms. When it is higher, new sensors are needed. These can be acquired at your alfa or fiat dealer for about € 90,- each, when they ask more you are ripped of.
For more infos on selespeed look on Selespeed Anleitung und Dokumentation
This guy helped me to understand the selespeed.
The GTA has another system the the 2.0 but a lot functions the same way, the GTA is however similar(but not the same) as the system on the fiat stilo.
I own a GTA selespeed now and sometimes it doesn't declutch in first and second, i believe this has something to do with the brake-switch, because i get an error sometimes hitting the pedal from cruisecontrol and then the cruisecontrol won't work also i hit the pedal again everything is ok again.
Please keep us informed of your findings.