put the line reset(player,POLYGON); into the tanks main action, just before its while loop.

Also, turn off (temporarily) any shaders the tank may be using.

4600 polies isnt very big...
Roughly how big is the MDL file itself? 1 meg, 5 meg, 25 meg, 50meg, etc.

Maybe its got a loose un-attached vertex somewhere below ground level...
Run it, hit F11 twice and look at the Blue bounding-box. Does it penetrate the ground by much
?