Don't touch any yellow cables you find: they're for the airbag system!
This problem is quite common, especially on older GTV 3.0's. The throttle potentiometer and its' connector are located above/behind the fusebox. Firstly, locate this connector and clean with a decent electrical contact cleaner. If that cures the problem, then great, but if it comes back, it may be necessary to cut out the connector and solder the wires permanently. If cleaning the connector has no effect, check the wiring below the heater unit, as some versions have another (similar looking) connector here.
If the problem is still present, you're going to have to do a continuity check from the throttle potentiometer connector to the plug on the ECU; some cars have a problem where the wiring loom is pinched behind the heater box (original assembly fault, and only relates to earlier cars with the ECU in the passengers' footwell).
If all that fails, then it's likely that the potentiometer itself is faulty, and requires replacement.