Your ambient is in line as well as your glycol temp and chiller operation. I would look at the contact between your coils and the conical.
I you have not seen them I have a fairly detailed writeup of how I assembled everything on Flickr linked below. In this pic you see how I used wood blocks at the bottom to properly place the coils. You see the ratchet straps that I used as temporary attachment points. I would wire the copper to the ratchet strap and then TIGHTLY wrap the copper tubing around the vessel making sure that it made good contact. When you get to the end you take the second ratchet strap and wire the other end of the tubing to that so it stays tight. THEN I took the copper wire and forced it in between the tubing. After that was in there and tied off I took clamps and pulled the coils as close together as I could and then wired them all together.
If you think you could get some of the slack out of your current coil it may be worth trying that first. If you could find something to place in between the coils it may also help the transfer. Then insulate it well. You could switch from the Reflectix to Armaflex like B3 uses, I'm sure it has a much better R value.
But then you're already 35 degrees below ambient which is pretty darn good and you only have about a 10 degree differential between your glycol and the beer. So we're getting pretty close. That being said you could youse an antifreeze that goes to a lower temp. Something with a higher propylene glycol content or switch over to ethylene glycol.