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I am certainly not an expert and not familiar at all with the European config you refer to of the TB on top of the intake manifold, nonetheless your problem of a surging idle does sound like a problem with the idle air control valve (which is controlled by the throttle position switch you mentioned). If you previously had an intake manifold gasket leak, the engine could have been able to get all the air it needed at idle through the leak (probably would have idled high too) but now that the leak is gone it has to get its air properly. At idle, the airflow is controlled by the computer activating the idle air control valve. The computer is able to tell that the engine is at idle by the throttle position switch you referred to on the throttle body
First check to make sure the throttle position switch is mechanically functioning correctly by listening/feeling for a click immediately upon opening the throttle. Do this without the car running so you can hear/feel the tiny switch when you rotate the throttle slightly simulating applying pressure on the gas pedal. If you hear the click then the switch is at least functioning mechanically correctly (someone else would have to help you determine how to test its proper electrical function if necessary). The function of this switch is to indicate to the electronic control unit (the computer) that the system is at idle and the computer then uses the idle air control valve to control the amount of intake air for a smooth idle. The idle air control valves can become very dirty and gunked up, especially if the flame trap has not been kept clean, and often just needs to be cleaned out with throttle body cleaner. Check the 700/900 FAQ section for much more detailed instructions on doing so. There are also very good instructions there on how to adjust the throttle position switch if you find that it is not properly working.
Jim
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