Has anyone else seen this (Poser 7, in my case)?

After a scene render lasting six hours, I spotted a small detail needing correction (doesn't that always happen?), so I started a partial render.   For an age, nothing much seemed to happen.   The render bar didn't budge and the task manager suggested that Poser was taking all of the CPU, without doing anything with it.
Memory was staying at a constant level, which generally indicates inactivity.

I minimised Poser, to see what happened.   CPU stayed at 99% and memory dropped to negligible levels and stayed constant - again, no sign that anything was happening.   Then I maximised Poser again and suddenly a bit of the scene appeared.   Thinking that the rendering was finally underway, I left the thing alone, only to find, ages later, that absolutely no progress had been made.   Repeating the minimise/maximise thing, I again found that the next part of the render was executed straight after the maximise.   After that, I completed the partial render quickly, but Poser only rendered a bucket after the minimise/maximise operation.   After it rendered a segment of the area, it seemed to go to sleep and it was as though minimising and maximising kick-started it each time.   The partial render can't have been more than 2-3% of the total area, but took something north of two hours, mainly because I didn't realise at first that it wasn't doing anything, or how to make it do something.

Oh - and the render bar stayed at zero all the while.  I saw the earlier thread about the render bar in P7.   Previously, I'd had the problem with the bar legging it all the way to the right when the image was only a third completed.   This time, the bar just wasn't coming out to play, at all.