JMPR,
The situation you're encountering is rather atypical. Many of the render issues I've had or reported here by others have been either with a particular project rendering to X point at which it just spins its wheels (almost always a corrupted file/clip header or something glitchy in the project file) or simple slow rendering in general. This last one is something that can take some working through to make sure all the appropriate settings are set appropriately.
Vastly varying render speeds within a project is quite unusual, unless one has between renders done something major to add or remove GPU/CPU intensive effects. So ... I do think it reasonable that a solution can be found for your ish, but it might take just a bit of patience. And Kevin is quite reliable in rendering assistance.
neil