Let me guess, it got worse after you had the radiator flushed. The folks that flush the radiator like to put in little gummy particles to prevent leaks. Unfortunately, your radiator may have been flushed several times before and is therefore partially blocked. With a conventional radiator, this only slightly lowers the cooling efficiency. The XJS V12 uses two inlets and devotes the top 1/3 of the radiator to the left engine bank. As the left bank water flows across the radiator the now cooler water must blend with the hotter water entering from the right bank. Normally this doesn't cause a problem when everything is new and working correctly. However, once the cooling performance drops, a over heating situation quickly develops. The hotter water from the right bank will develop a higher pressure than the left bank water which prevents circulation of the top 1/3 of the radiator. The left bank no longer has cooling and overheats.

The solution; rod your radiator, put in a new core or get a new one. I opted for an aluminum AJ6 radiator and modified the two inputs to only one. I had no problem with 104 F Florida heat. My cost: radiator - $250, piping - $100, labor - one weekend, experience - priceless.

 

If there is air in the system it will cause the engine to run hotter. I had a radiator hose leak in the middle of the Australian outback a local friendly mechanic replaced with a modified Ford but did not bleed the system as per the book, as a result I had intermittent overheating.

The only way to flush the radiator is to remove the end tanks and brush clean it

 

The radiator is a prime suspect, yes. And....it can be externally clogged with leaves and debris. You'll be shocked if you pull it out.

How's your fan clutch?

How have you bled the system? Did you elevate the front of the car and get a strong flow from the bleed hole?

Post back.

Cheers
DD