Incandescent bulbs change with time, even halogen bulbs. That is why I asked about the consistency of corrections from one session to another. Are they always in the same direction? It seems that the only possible causes are the ones you are either reluctant or unable to measure. Surely you have a light meter for use with your camera. At the beginning of each session and at the end, remove the enlarger lens and measure the incident light at the lensboard. Whatever the ultimate cause of the inconsistent prints, this measurement will show if it is the light source.