How bad is the uneveness? Remember that these cameras were the point-n-shoots of their generation and weren't build to Leica standards. Some uneveness is probably normal. For what it's worth, I also have a couple of GIII's which are fairly good, but not great, regarding frame spacing.
The uneven frame spacing follows a pattern. It may start with normal spacing; then got to narrow spacing; then back to normal spacing; then go to wide spacing; then back to normal spacing. The pattern then repeats itself.
The narrow spacing is so narrow that there is no separation between frames. In fact, there may even be slight overlapping of frames. The narrow spacing poses a problem for me when my automatic film scanner cannot detect a space between frames.
As this happens in two cameras, it makes me think that this might not be a camera problem. Are you sure you leave the film with the perforation holes well placed in the film transport pins, before closing the back? And do you pull out the film far enough? (as far to the right as to reach the orange line)