Yes, the roller may be the issue, as the tech suggested I give it a spin to make sure it's rotating freely. It does rotate pretty freely but stops from its own friction fairly soon. Seems ok. Tech agreed it might not need service even with the roll counter reading 595, and asked me how much I intended to use it. He also said the shutter is good for hundreds of thousands of clicks. I didn't ask about the tinny sound. 🙂
I do habitually put some drag on the fresh feed roll as I get the paper leader started winding on the takeup reel until the arrows are lined up. Good to make sure to get the roll started tightly to avoid binding later from a loose wrap or light leaks after unloading.
Thanks for your observations. If this is normal, I guess I can live with it, but it seemed unusual to me, having experience with two other Fuji's (645's), Bronica RF645's, and Pentax 6x7 and 67II. That 67II is particularly precise with very even but tight frame spacing to get 21 shots on 220. None of these other 120/220 rigs have anywhere near the variation in frame spacing the GW670III shows. And I will run a roll of 120 to see if that feeds more evenly.