I don't use TSM Personally but I believe you can use it to modify track magic data, if not you can use the CMagic Tool that's available in most GP4 places, and use it to extract and edit a track's Magic Data. In the magic data is a line called "Air Resistance", which should be around 18000 I think for Monza.
If it's not happening to your car though it sounds like just the AI Setup, or perhaps their CC Power Factor... try and find the Magic Data for the Track using CMagic or TSM and check the Power Factor is around* 500. It could be their setup too, the wing might be way too high for the gear ratios. Explore the Track Magic Data though to check the AI Setup against your own, and also the power factor maybe..