It's Alive

I have a Delta single handle pressure balanced bath valve (model 1700) in my shower stall that is 8 year old. The problem is that is drips water (at a fast rate) about 5-10 minutes after the shower is turned off. The dripping lasts about 1-3 minutes and then stops completely. I have replaced the seats, springs and seals and checked the cartridge assembly (shaking it and listening for rattle). It still drips. Anyone have an idea of what might be causing the dripping?