If I had to guess, I would think of two, maybe three settings who can cause this behavior. First thing is the automatic save function in the Poser preferences. It might kick in by chance at the very moment the image is finished rendering. You mentioned that this behavior is not always present, therefore it could be a time bound process in Poser, like said automatic saving. Second suspect is the Nvidia filter in the Render Settings. Poser creates two images if this option is on. Maybe sometimes Poser needs more time to process this filter onto one of your images. No. 3 on my list is lack of RAM or a defective RAM Bar on the Mainboard or graphics card. This would mean that you would notice slowing down behavior on other software too. I would run some benchmark tests on the graphics card and Mainboard separately and look at the log files. If something wrong with parts of the hardware, it will show up.

I hope it's just a "wrong" setting. Keep us posted on this, it might be useful to others as well.