Understood.
There is something with your screensaver going on that's causing the CPU to be running, otherwise your temps would stay the same. Since there are more screensavers out there then there are sand grains on any beach, there's no way to tell.
The important point is that your CPU isn't running at idle, when your SS is on.
Pretty much any heatercore will do you a world of good. I don't know what kind of heat a P3 gives out, but I'm sure that with a heatercore, you'll be able to achieve temps near ambient, and probably below, regardless of the load on the CPU.
Did you ever check the temperature of your water? That alone should give you a good clue.
Here's an
article on radiators, from Bill Adams. You don't need all the theory, but I think you'll get the point.