Recently, about 30 minutes into driving, an amber warning light comes on that says Failsafe Mode. It still drives, but Cruise Control won't work and that light goes off on the J-gate console area.
It doesn't seem to affect my driving, but obviously I want the warning light off and to make sure I'm not causing further damage by continuing to drive with it on.
I'm a girl with limited mechanical experience, so simple terms/descriptions are appreciated

 

This fault will have thrown up one or more fault codes. These can be read by a suitable reader at a Jaguar dealer, or a Jaguar specialist. Faults are of two kinds, those regarded as very important will not disappear until the system is reset. Those of lesser importance stay around unless the condition that caused the fault disappear. Normally it waits for several engine start cycles before deciding to cancel the fault warning. However the actual code will remain in memory.

An Amber light plus message is not as important as one showing a Red warning light. It sounds like you have a transmission data problem. Any recent work carried out at all ?

 

No recent work done.

The message turns off when I shut off the car.
And it doesn't always come back on- only when I drive it again for over 30 minutes or so. Then it shuts off again when I turn the car off. Literally, I can pull over and shut of my engine for one minute and restart it with no problems, no warning message and my cruise control working again.
Also, if my trip is quick (like 15 minutes for errands or something) it may not come on at all.

I thought I'd try posting for help here before taking it to the shop. I've often found the solution is simple and I can do it myself without paying a couple hundred bucks for them to just look at it and come up with the same thing.
(*Side note- I only do this for minor things, not red-light warnings or when there's weird engine noises or anything.)

 

You might get some more helpful info if you post in the X300-X308 section, which covers your model. This is the X350 2003-2009 section.

 

In the US, you can go to an Autozone or Advance Auto store and most will read your fault codes for free. Write them down (the codes, not the descriptions, since some Jag codes are unique) and post them here.

You can then get much more accurate advice.

You do have an X308, not an X350 XJ model...keep an eye out...one of the moderators may move this post to the correct section.

 

Oops! Thanks for the correction. I'm still a newbie to posting, so I apologize for not knowing exactly what I'm doing quite yet.

 

Well, the best thing to do is to acquire a scan tool and learn how to use it to retrieve codes.

That is the first thing, and a required thing that a dealer will do. You might as well know what they are going to see. Armed with the ODB-II listing you can cross-reference to the possible causes.

This one is suitable for a budget concious beginner:

ScanTool.net LLC - ElmScan 5 Compact -- ELM327 based scan tool - ScanTool.net

$24.95 with included software ... the software costs more than that if bought separately.