Considering that you have exposures on every other frame, that makes me think that the back is defective, not the winder.
Take the winder off and do another test roll. You can just slide the pin back in place and twist the wind knob on the body easily; no need for a handle.
At least this will tell you if the winder is causing the problem.
All in all, the wind handle and film advance are both pretty simple mechanical devices. No tricky electronic couplings or such going on. The Speedgrip has only an electric hot shoe connection; all the other connections are mechanical. The wind lever on the grip is simply doing what you would do directly to the wind knob on the body.