I have to disagree on one point. My Type 4 wagon acted like it had a sail. A heavy gust once blew me right out of my lane on I 95 north of Philly, Under less severe circumstances it tracked perfectly.I would very surprised that any VW would be unstable and any attainable road speed. VW designs all their vehicles to be suitable for the European driving environment, even if not sold there. All my VW over the last 40 years have tracked like an arrow with very little input from the driver other than to deviate from straight. The Mk7 Jetta is far from being small and does not have a soft suspension. Never had a VW other than my Vanagon that "gets pushed around by the wind"
You need to check your tires and that they have a proper pressure.