A couple of weeks ago, in slow-and-go traffic, I suddenly got "GEARBOX FAULT" all of the gauges went dead, I got the battery warning light, and I only had top gear.

Being 300 miles from home, this was an annoyance. I pulled over, shut off the engine, then after a couple of restarts and retries, the problem was gone. I got home and it was running fine, so I figured it was just one of those things.

Then a week later, coming off of the freeway after a normal trip, same thing happened again. I got home and haven't been in the car since, so don't know if it cured itself again or not..

Before I start digging, has anyone seen this before?

 

Yessir. In my case I had some very hard/abrupt shifts randomly for weeks. Then one extremely rainy day Gearbox Fault, a lot of seemingly random shifts, then only high gear so very difficult to drive home. Drove it a few days later and no problems at all. Later driving after another heavy rain, Gearbox Fault. That time the CEL came on and gave me a CAN bus fault and trans output speed sensor error. I did some searching here on the forum and decided to check out the trans control module and sure enough found water coming down the wiring harness and into the connector had corroded a few pins. I cleaned those up and put some RTV around the wiring as a dam to prevent water getting in there again. The root cause appears to be the cowl drains plugging up over time so water can pool up high enough to reach wiring going through the firewall. When I opened up the cowl I found years of dirt, leaves, twigs, and a big mouse nest. I poured a big cup of water in there and it must have been a minute or more before a single drop came out the bottom onto the ground so, yeah, clogged drains.

https://www.jaguarforums.com/forum/x...4/#post2913567

Do you still do keys? I'll DM you.

 

I'll check into that, but both of these happened in warm dry weather.

Yes, I still do keys.

 

I am getting close to solving that issue on mine so look this week for the ronks2 name and follow along.

Couple quick things to check
All of your abs exciter rings even if there isn't a code showing it both my front ones are gone and no code for the last year

Warm the car up and get a laser temp of the battery cables and see if one is alot hotter
Have your battery checked at advance Auto parts. Mine held 12.7 + But still failed.

Make sure your TCM is dry
God luck and follow along my post