VAG-COM: Connection lost above 120 km/h?

After a friend lent me his (I'm guessing it's a 'Noname' interface with Optos) device, and I found a colleague who has VAG-COM, we went for a diagnostic session today. To my great surprise, I discovered that: above 100-130 km/h, the interface stops providing data.

The connection is lost, and no data is being transmitted. However, there is power, as indicated by the interface LED (which is lit and blinking...).

Since my car also jerks during acceleration (which is why I had it diagnosed in the first place), I suspect the infamous '109' relay. Should I replace it tomorrow morning just to be sure?

'There were no relevant error codes in the memory; only a 'load signal for generator terminal DF - implausible signal' that has been there for ages and I should probably clear.'

The following were used: A full version of VAG-COM 311.2-N (if I remember correctly), Windows XP, an interface (someone might recognize it, it's an integrated RS232 to USB converter with a red casing and a transparent body), and my laptop.

Car: VW Passat, year 1995, 1Z engine, 66 kW.

If no one else has any ideas, I'll probably change the 109 tomorrow.
Thank you and greetings.

Try using a proper interface, without a USB-to-RS232 converter. Ideally, use something reliable (like HEX).

'Where do you get it if not by stealing? *sigh*... I already had a lot of trouble borrowing this thing (Unfortunately, I'm relatively (very) inexperienced when it comes to car repairs, and I just need to find this intermittent fault because my workshop is unable to do so after 3 attempts ).'

Okay, so I'm looking for someone from Linz or the surrounding area who could lend me a diagnostic device for about a week or two (of course, for a fee). I would be very grateful. Because as a student, €250 for a diagnostic tool that I might only need once is quite expensive.

'Thank you!'