Small Coolant Leak

I have a 1993 JGC with the V8 engine. Recently it has begun to leak a fluid
out of the radiator cap. It only leaks fluid after the engine has warmed
up, and is cooling down. I was able to follow the trickle of coolant to the
radiator cap. At first I thought that the cap was bad, so I replaced it
with a new one from Checker Autoparts. That didn't seem to work, so my next
thought was that it is building up too much pressure. The only problem is
that I can't think of a reason why it would build up extra pressure. Is
this a possible sign that my water pump or radiator is dying? Or, does
anybody have another idea.

The only thing that has changed on my car was that I had the oil and
transmission fluid changed before I noticed this problem. Also, I had a new
radiator put in 2 years ago, and a new fan put in last year.