Was just running a mission with the Marder I and was able to traverse the main gun with no problems.
I have my JSGME set up for the game as follows... Playable Panzer II, III & StuG v0.2, Unofficial_patch_SF_v0.1_eng, stugIIIG_Mod_v1.0, Steel_Sound_Mod_v1.1(fix)_eng, Steel_Panzer_Mod_v1.5_beta and SPM_V1.5_beta_English-patch.
A couple of screens taken from the gunner's position and traversing the main gun while stopped and firing on the enemy tanks.
http://img.photobucket.com/albums/v4...07_24_0000.jpg
http://img.photobucket.com/albums/v4...07_47_0001.jpg