Danny, don't know much about the XJ6, but my hesitation issue (on S Type) turned out to be Coils.

The symptoms where almost like clogged filter (and sort of like slipping trany too) but turned out to be coils. Took 3 weeks for a code to show up.

Good luck.

 

You may also want to remove the throttle body for a good cleaning.

 

That's where I would start. Throttle body cleaning and then I'd go with the coils. Fact is, a clogged fuel filter will start to affect performance over time, not in the span of a week or two. It takes using the shittiest gas and not giving a crap about your car to get a fuel filter clogged in under 80k miles. I doubt you'd let it go that long without changing it would you danny. For sure it sounds like a coil problem, but before ditching out 60 bucks a pot on coils, I'd clean the TB. From there go with injectors, but then again comes that issue of time and how often do injectors go bad. The whole thing is being honest with yourself. Do you treat your car like crap and run it like a rally car. If so, then fuel filter/injectors could be the problem. BTW, does the car idle fine. Because if it doesn't, the I doubt it's coils. I would point in the direction of vacuum lines in that case.

 

does the XJ6 use a Cap & Rotor in the ignition system? I've had a hairline fracture in the cap cause hesitation and a worn rotor (60k miles) cause it in previous cars. Just good tuneup things to check in the process IMO.

 

Individual coils on that model, but you bring up a good point. Pull the ingnition coils and inspect the boots for cracking. You may also want to check the valve cover for signs of arching. Also, pull the plugs and look for signs of oil on the outside of the plug. This would be a sign of the spark plug boss seals leaking.

 

Back to your original question.....the fuel filter is located right next to the driver side, rear tire. If you suspect that the fuel filter is the problem, it will only cost about $10 to replace it and find out -- worth a try. The worst that could happen is that you now have a new, clean fuel filter -- oh well!
Others folks suggested replacing your coil. If your 97 xj6 is like my 96 VDP xj6 (and I'm 95% sure it is), then you have a coil-on-plug system, where each cylinder has its own coil (attached to the spark plugs, hence no spark plug wires). I doubt that your coils are the problem, but who knows.
I would suggest changing your fuel filter ($10) and you spark plugs ($10-60). These are cheap, fairly easy, regular tune-up procedures that can only help the overall peformance of your car. My new plugs gained me about 1.5 mpg, which may or may not just be a placebo effect. If you do take this advice, then here are a few tips:
--be carefull loosening the bolts on the fuel filter. Mine were rusted onto the filter, and I ended up having to cut and repair the fuel line (nightmare!). You might want to apply some rust-eating solution to the bolts before you disconnect them. The 96 xj6 actually had a recall problem with the fuel filter (not made to specifications), which I suspect is why I had to cut my fuel line getting it off. Hopefully this is not the case with the 97. If it has been changed at least once since 1997, then you are probably OK. I'm pretty sure the fuel filter that I just took off was 11 years old. Also, you'll have to jack up the car and use jack stands to change the fuel filter - it's just too damn low to the ground otherwise.
--spark plugs - buy NGK iridium IX plugs - a little more expensive, but pretty much the best plugs you can buy. 1990's jags (and possibly newer) come with Champion plugs, which are fine as well, but I've heard from several jag experts that NGK are the best plugs for a jag. Make sure that you carefully gap the plugs before you put them in (probably to 035 (.889mm) since we have the same engine, but check your specs first). Even though they claim these things are "pre-gapped", they most certainly are not. If you don't already have a spark plug gapping tool, buy the nice one ($5), not the $1 cheapo at the counter. Make sure you have a spark plug socket, and an extension bar for your socket wrench. To change the plugs, you'll have to remove the engine head cover (the one that says "jaguar" on it, just 3 bolts) and then take off each of the six coils covering the spark plug holes.
My apologies if you are already fully aware of how to change your spark plugs. Good luck. Let me know how it progresses.
I've been told by others that our AJ16 6 cyl. engine is the last true engine made by jaguar (without significant Ford interference, which started on the 8 cyl engine). I don't know this to be true (100%), but I'll keep believing it until proven wrong. Regardless, these engines are smooth and fast as hell, and I hope you are enjoying yours as much as I am enjoying mine.
Cheers