On/off not functioning with the head screwed down. If I monkey with both ends long enough, it can work, but it irratic.
If the bulb housing is removed and the bulb is pressed to the connection, on/off functions fine.

Options? Replace the whole head/bulb? Replace the bulb housing? Anyone else have this issue?

What should I replace it with?

You probably need a outer spring as a spacer.  It would go around the brass pill and give better contact with body.
You would be better off with standard P6 host.
Lighthound has Lumensfactory warm LED that fits my Z2 better than Malkoff.
My non Malkoff or Lumensfactory hosts are Solarforce.

Also make sure the pill is screwed in tight if it isn't a Malkoff or Lumensfactory drop-in.

I would upgrade. Much more efficient options are out, now. EDC+ and Malkoff are the nicest I've seen.

I borrowed another g2 and mixed components with mine.

It's definitely the bulb.

Thanks for the above recommendations.

I took the flat oh ring from on top of the lens and moved it on to the top of the LED assembly and it was just enough tension to restore reliable operation. I also had twitch on/off and just plain not working with my LED upgrade. The outside spring won't work because it's just too damn long. Maybe if you clipped some coils?