I will show this thread to my tech guy here this weekend if he comes in and will fill you in on the cleaners we use ect.
Are you using Kodak ,Fuji, Agfa chemistrys?
Are you using automatic replenishing on the machine or manual?
Is there a long period of time between printing sessions?
Have you checked manually the tempeture of the developer and the bleach fix after the machine has been turned on.?

Immediate colour/density shifts leads me to think : voltage, dichroics, fogging problems.

Slow colour /density shifts lead me to think: contanimaton chemistry, improper mix, insufficient replenishment volumes, variation in chemical tempetures.

I do not think it is the paper as I think you mentioned that you have tried new paper . is that correct?

If you have thouroughly washed the racks , flushed the tanks and recirculation pumps and hoses as well changed the filters, I do not think it is your cleaning proceedures.

Honestly I think it is your dichroics / timer/ voltage/ or plain mechanical fluctuations of the enlarger.