Please.......PLEASE, tell me your S-Type is not British Racing Green.
Let me tell you a story. There is a certain 2005 S-Type that has been in my care since it was new. Long time customer that has had Jaguar's from us since the Series III, maybe before. In 2005 he replaced his '97 XJ6 with this S-Type 4.2L. This car lives entirely here in the Midwest, and from the onset, like clockwork, in November he arrives with a Check Engine light and misfire fault codes. They continue returning until late March or early April when the fuel blend returns to a normal blend. He's fine all summer. He was here last in early December for a coolant leak repair and odd stuff. He left here 12/5/14 with 117,800 miles on the clock.
How's that for deja-vu??
This is the only car of that vintage that is still with us regularly, but I can tell you he is not the only one that exhibited this scenario. Why only the 2005 models? I can't say. I can tell you during the period where these cars were 2-4 years old I worked with Jaguar on numerous occasions and the tack was always the same. We would try this, try that and never really put a thumb on it. Then March would come. Problem solved. :-)
After a time, when these cars were basically out of the warranty period, corporate, which was Ford at the time, pretty much lost interest because dealers saw very few of the cars anymore. To top that off, all the cars got a new engine management system for 2006; so the attention pretty much went to them. Of course, the new system also did away with the misfire oddity.
To cut to the chase, the only real impact we ever made on the issue was moving to a fuel that had no alcohol at all during the winter months. We hit on this when a fellow who managed a large farm used co-op gas that was straight gas, no alcohol. He had several winters with nary a Check Eng light. A few others we recommended that to also saw improvement.
As far as any technical explanation, the best I ever got, was it somehow related to the change in volatility and vapor pressure that came along with winter blends, combined with ignition mapping in the engine system. Doesn't really explain much, does it??
Not really what you wanted to hear, I'm sure. But that's the best I've got.
Cheers,
And Merry Christmas, everyone!