Just thought I'd recount my experience repairing my JTV89 which had an unreliable VDI connection pretty much since I got it. This guitar was an open box from Sweetwater so I saved a couple hundred bux on it. I was sure it wasn't the VDI cables since I also have a Standard and the cable works reliably with that guitar and the Helix. I figured the pins in the RJ45 jack in the guitar were bent - often I could get it to work by fiddling the VDI jack.
Full Compass is a great source for parts for the JTV and line 6 stuff in general. I found a replacement input jack there - $52 seemed like a lot, but shipping was free. The 69 jack is slightly less expensive. Shipping was pretty quick and I had the new device in about a week. To replace the jack, I needed to open the back as well as remove the four screws securing the input jack. The Jack assembly has to connections, a six or so wire electronic PIN socket and a ground wire which screws to the guitars ground. No soldering is required. Disconnecting the PIN socket required a needle nose but came out pretty simply. The new one only goes in one way.
I noticed that the old jack was not grounded- the green ground wire was not connected to the ground on the jack - it was loose connected to the guitar. This might explain the defect, but I'm not sure why it was intermittent. Rather than trying to re-attach the ground to the old input jack I went ahead with the replacement because I was unsure of any other issues, and i already had the replacement part. After the replacement, which took about 15 minutes, the guitar worked perfectly again.
The component based design makes the JTV a very serviceable guitar. Of course, on a standard guitar, the wiring is far simpler and repair might be less expensive.