Curmudgeon10 discomacc45
Coming here late to say that the same problem occurs on the Roku Express 3930, manufactured in 2019/2020. And I read a lot of people online with questions similar to this even on 4K models, including the wired versions, which in my opinion rules out the hypothesis of outdated models and lack of support.
I've been experiencing these problems since November, and neither Amazon nor Roku support can explain (or solve) it. This last one barely has accessible support, btw, with an email that doesn't really answer your questions, appearing to be all automated.
My internet speed is always with an excellent signal according to the device itself and my model does not have an ethernet port.
I rarely have problems with other apps on the same Roku Express, and Prime Video has never given me a headache on other TVs and devices. It appears to be a specific problem with Roku > Prime communication or vice versa. After almost 7 months I simply gave up. Sometimes I can watch an entire movie, overnight, for some reason. If the settings of my router wasn't blocked by the ISP itself, I would look deeper, but I suspect the result would be the same in the end.