Hum in phono that comes and goes

There likely is some sort of grounding issue. Have you checked the tonearm cable ground connection at the preamp to make sure it's tight? Has anything been changed around lately in your system?

I have checked the grounds, but I don't think that is what it is. I would think that a faulty ground would hum all the time, which this does not.

Ground loop-induced phono hum can be intermittent and mysterious. Try running the phono stage power cord to a different outlet or circuit to the AC mains.

Again, it doesn't act like ground loop or faulty ground hum. Since touching the tonearm repeatedly will cause it to diminish gradually until it's gone. It's like something is getting charged up sitting, and releases when you touch the arm, or the arm is playing.
I've changed outlets AND circuits, as well as power isolators, phono cable, and arm without any effect, I figure it has to be the cartridge or phono amp, with a slight chance of it being the pre. I'm going to change the input on the pre today to see if that has any effect.

Tough problem. Just a guess, but I would suspect the phono stage or its power supply. And a conceivable scenario would be that a capacitor in the power supply that is connected between AC "hot" and chassis/AC safety ground for purposes of filtering high frequency noise is electrically (not physically) leaky. If so, the transient caused by touching the arm, especially if that is done at the same time as touching the preamp (which would provide a path from the tonearm to AC safety ground via your body), momentarily affects the phono stage grounds and "shocks" the capacitor into some semblance of normalcy, temporarily.

As I say, though, that is all just a speculative guess.

It might pay to open up the phono stage and its power supply and examine the capacitors for discoloration or other signs of problems. The kind of AC filter capacitor I'm referring to would be located near the AC input connector of the power supply, and would NOT be a big electrolytic, although it would be a good idea to examine those as well.

Beyond that, I'm not sure what to suggest other than trying a different phono stage if possible.

Regards,
-- Al