My Fiat Tipo DGT Speedometer doesn't do it's job all the time, it only works correctly for small periods of time Does anyone have a possible idea for the reason that makes it work like that?

Sounds like a connection problem. Check for loose wires and connectors.

Open your bonnet and Check on top of your Gear Box, there should be a plug (there is on my one, if not you might have to go behind the dash)

This is where my one plugs in, it was loose and a lil fiddle with a long slim object to adjust to connecters might fix your problem.

yes,runs off a sensor off the gearbox.proberly as already menthioned a bad connection.