Paul,

First things first: thank you for the membrane switches and connector contacts!  You've helped give this old beauty a new life.

I do have that same MIDI interface.  Thanks for the tip on its issues... luckily I don't use it very much as a have a Midisport 2x2 that's my "main" interface.

I have no idea why that interface would draw power off the MIDI output but it definitely does.  With the interface not connected to a computer, if you plug its input into the synth's MIDI output, the red LED nearest the MIDI cables on the interface lights up.

Anyhow, disconnecting pin 2 on the Polaris MIDI output from chassis ground and connecting it to digital ground via pin 5 on J11 on the main board completely fixed the "clicking" issue.  Yay!

Anyhow if anyone's interested and hopefully this doesn't violate group policy, my Polaris is now for sale.  Price is $950 but I can come down a bit for members of this group.  Local pickup only in Wilmington, MA.  You can read more details here: favorite this post Fender Chroma Polaris synthesizer - $950 hide this posting unhide
Oh one other thing... I've seen some people on the internet comment on something that bugs me a bit with the Polaris - that the pitch-bend lever works "backwards" compared to most other synths.  It looks to me like that could easily be changed by swapping pins 4 and 6 (orange and brown wires) on the plug that goes from the levers to the left control board.  Has anyone ever tried that?