Frequency dependent resistor in real inductor (air coil)?
I made a little FEM simualtion of an air coil with paramters as you describe (5 cm height and width, 0.6 mm wire diameter copper, L = 5 mH, Rdc = 5 ohm, N = 450 in 6 layers). After 10 kHz the resistance increases significantly due to skin- and proximity-effect, see figure. I guess proximity effect is a main contributor.