GM made a bunch of steering racks that exhibited this problem. It was commonly called "Morning Sickness." The cure was a new rack.

I would suspect that the valve is sticking. It is also possible that the buffering devices in the lines (located where the lines bolt to the rack) are not letting the cold fluid through properly.

If it were mine, I would try to change more of the fluid. The reservoir contains only a small fraction of the fluid in the system. Most of the fluid is in the steering gear. Use the MB stuff.