It’s simple the end of the line for this engine, they took a 215 cu in motor bored it to 240 cu in and run it 30 degrees hotter.
At some point you have to stop boring or you don’t have enough material to disperse the heat or hold a sleeve in tight.
You dont hear of these problem with 3.5 or 3.9's, it's usually the 4.0 & 4.6 that have problems.

Gm took the motor all the way to 300 cu in, but they went back to a cast iron block with alumium heads

 

Makes sense..

 

manufacturing process changed. BMW and then Ford.
Older Range Rovers had this problem too.

Here is a nice tidbit

Range Rover Parts Engine : Land Rover Engine : Used Auto Parts

 

Something definately changed, because they f***ed up the dowel machining on the front of the engine for the "2003 model year". This wouldn't have happened unless they had put it on a new machining centre, modified the machining program, or god knows what else... Basically, we know (for a fact) they went in and modified the programming creating some of the machining on the block casting, so it is entirely feasible that they also modified other machining patterns (such as the bore fits).

Also, the cracking of the block at the thin section between the liners (resulting in coolant loss) was probably starting to really become well known by this point, so (again, my opinion) it is entirely feasible that they decided to reduce the interference of the fit between the liners and block in an attempt to lower the hoop stresses on these weak areas, thereby reducing the risk of cracking. They just pushed the envelope too far, however, allowing the liners to move after the engine had gone through many thermal cycles

There don't appear to be any facts to back any of this up - but it perhaps offers a somewhat plausible explanation as to why 2003> engines seems to suffer (note that the 4.6 was only sold in USA, but in 2003 you could still purchase a 4.0 in the UK - but they were sold in VERY small numbers - like less than 1% if you look at the volumes available on the used market).

 

all the more reason to run a lower temp stat