Does it do it on the first flight after loading FS
I was messing about practising takeoffs in my PMDG 737-700 and after starting a flight, the plane became uncontrollable - one time it veered to the right and other times it took off on its own, accelerating to Vr and climbing. I guess that after the restart the autopilot was not disconnecting itself and was trying to turn to the selected heading (360 degrees by default).
Do the planes veer off on takeoff, and is it towards the selected heading?
If you take off in the opposite direction, does it veer to the other side?
Another problem is that when landing, some planes still try to keep the localiser heading, even with the autopilot disconnected. Once you've passed the localiser on landing, it tries to turn around.