Your expertise would be appreciated

I recently purchased a VPI Classic 3 Sig. SE. I decided to try a couple of new phono interconnects as well. I purchased the VPI and a Bob's Devices mid-line, (the BD's internal construction doesn't allow the ground to pass completely through, not sure about the VPI).

I am using an EAR 834P w/volume and a Bob's Devices 3440A SUT into the MM section of the EAR with a  Dynavector Karat D3, Ortofon Cadenza Mono, Acutex 420 STR. The Dyna has around 600 hours, the Ortofon is brand new, the Acutex is just back from being re-tipped.

When I had the Dyna and Ortofon installed, the VPI introduced hum into my system. I tried running a ground wire to/from every conceivable location and nothing changed. I then installed the Bob's cables and all was good. (I sent the VPI back).

Yeterday, I reinstalled the Ortofon and got hum. I then installed the Acutex (of course I then remove the SUT) and quiet reappeared. I finished the day out in this manner.

Today, I start listening and the hum is back, only now, I can run a ground wire from the EAR to the VPI's ground lug and the hum stops.

Why does it hum on one day and not another with no changes? These types of issues are the most frustrating.

I don't know if this matters... I have two dedicated lines for my system. One for the front-end electronics, the other for my amp/subs. I have two Hydras, one for each.

Since adding the new TT, when I switch on the overhead light, (which is on a separate circuit) I get a very slight tick as I do when I switch of the TT.