Proceed GT bad straight run with you too?

Hello, I have noticed in the last weeks that my proven GT has a bad straight run. If I take my hands off the steering wheel, it always drifts left or right after about 2 seconds. In most cases to the right, because of the normal road gradient to the rain run. I drove a VW Sportsvan for comparison, which keeps its track perfectly. Then last week a brand new Toyota Corolla as a rental car in Ireland, which is also drifted left (left traffic) now my question, how does it look like with you? Is that normal, or should I go to my dealer. The Golf Sportsvan has only 195 R15 on it, which of course are not as susceptible to track grooves as our 225 R18. Greetings Jan