Carb Icing?
 

Are XT's known to suffer from carb icing? What are the symptoms?

Several times today my bike just cut out and died, as if I'd hit the kill switch, then sprang back into life a few seconds later (which wasn't funny mid overtake!).

Doesn't normally do that, but it's been sub zero temperatures today.

Never had carb icing issues on my XT, but I have on other bikes (Ducati. Honda): various symptoms from cutting out on idle, dying completely as you describe, and even carbs sticking open - really scary!!
In my experience it tends to happen when it is close to freezing, with lots of moisture in the air (mist/fog), rather than on drier, colder days.

Sometimes it's obvious ie the carb is covered in ice. Other times, leaving the bike standing for a few minutes is enough to thaw it through. Is that what you're seeing?

As XT carbs are rubber-mounted, and thus isolated from engine heat, they could be susceptible, but my 1VJ (with 3AJ air box) doesn't seem to suffer. Maybe the air is being warmed enough before it hits the carbs?

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I put a glug in a full tank. Its the freezing fog conditions that I find the worst. Fairly annoying problem on my Suzuki GS500.

could happen when your bike has been parked out in the cold AND you have water in your tank and/ or carb,
drain the carb once every month (in winter) and park inside (is always best remedy)
syrup oil is never very good on a single cylinder engine..