Thank you guys
The problem was with the plastic tracks, they bend easily on pressure. The metal tracks are rigid so the track guides do not move into the grooves. It's nice.
It's still not running completely perfectly straight but I accepted that already. Maybe I will buy a better transmitter in the future which has got trimmers, but it's not that important at the moment.
Anyway I'm thinking about buying a better battery, possibly a NiMH Battery pack between the range of 4000mah and 5000mah. What kind of battery charger should I get for that? I've got the asiatam lower hull from rctank.de with the special battery charger, which has a custom socket at the rear of tank, so I don't have to remove the battery in case of charging.
Any hints?