It's unclear, but from your first post it sounds like you're pulling them from the developer once the image is fully visible. The better way to do it is always use the same amount of time in the developer (test strips and prints) - 1 minute for RC and 2 minutes for fiber in that developer. That doesn't fix the issue of them taking longer each time, but you do need to start with a consistent process in order to figure out what is actually "wrong". The temp getting colder could well be it because that developer isn't likely to go off that fast. Also, if you'd used f4 for the earlier prints and f8 for later ones, as well as pulling them when they looked ok, then the later ones would appear to take longer.