Heheh it is a phenomenal car, im in the market for one (MY11) but waiting to finish one project first lol .....I would recommend a drive in the older RB26 (R32/R33 or R34 GTR) cars they are much more of an involved drivers car they are a handful. but not as quick as teh R35 GTR, you have to fight the skylines a lot more...
I think ive found the problem on his IS250 I think the water pump has shat itself or is on its way out...stuck a long screwdriver against the head, sounds sweet as a nut...
Stuck it against the water collector part and you can hear it clearly grinding....was dark otherwise I would have pulled off the belt and run the pump by hand to see what it felt like