What is the steering link to the tailwheel?There are a few different tailwheel steering mechanisms as we all know,ranging from bungees/springs to castoring/partially castoring.To fly a Thruster it is generally taught to get the tail up ASAP on takeoff as you would be aware Yenn.On some other aircraft such as the Citabria (and maybe the D18) it is recommended not to rush the transition to get the tailwheel off the ground.It sounds a bit like it was hard to control in the 3 point attitude with the tailwheel on the ground,suggesting that the tailwheel may not be aligned with the rudder.Think about if you were happily rolling with the tailwheel on the ground as your steering control and it was 10 degrees different angle to the rudder,then suddenly you lifted it off the ground,there would have to be a drastic change.If you combine this with the torque and slipstream effects as well,it may be quite significant.I could also suggest (as I once did in my Thruster),that you were not paying enough attention with your feet.I didn't groundloop but left the runway.You have probably thought of all these angles but I'm just trying to enlighten other readers of the many variables.