215 is supposed to be "normal" operating temperature for the AMC242 - it's spec'd for 210-215*F. Granted, this can be changed, with a little work (my 88 and my 89's all run down aroun 190-195*F, but I'm cranky.)
The relatively long cooling tubes in the XJ radiator do NOT respond well to buildup in them - flushing every two years is something I'd consider MANDATORY. Use a good strong flush the first time YOU are doing it (you don't know what's been done to it before then, do you?) and then you can probably get away with a gallon of vinegar and top of with water to flush afterwards.
If this is the first time YOU have flushed the cooling system, I'd also advise changing ALL the hoses AFTER you flush - you'll free up a lot of crap in there, and changing the hoses will help to get the rest of it (and, you'll know how long your hoses have been in service. I won't leave one go for more than five years...)
Also, if you start seeing "heat creep," note WHEN it happens. Typically, you'll see the heat go up at idle and drop at speed, which usually indicates a fan clutch. These are good for four years MAYBE, and are easy to replace.
5-90
