Trev,
I run level track on my shuttle, but I do have two cars and each are very different in performance. One ran perfectly out of the box. The other was a pig, requiring tear down and rebuilding, mainly due power pickup issues. I replaced the non-powered axles with Bachmann wheelsets as the stock wheels did not conduct power (even after only a few minutes running from new, the inner faces of the wheels had become hopelessly coated with carbon residue). From memory, I changed out the carbon brush pickups as well.
Mine do not slip, but once rebuilt have been quite reliable. Nice cars but would not recommend them. Check for drag from the trailing truck wheel pickups.
