Fan kicks in extremely late!

[PieEater]

I don't know if this is relevant but I had something similar inasmuch as coolant being forced out of the bike all over my leg with a RD350 YPVS this apparently was due to a cylinder head gasket failure,  I assume somehow this had pressurised the cooling system. I know that you have had a major rebuild but was the head checked to make sure it wasn't warped at all before it went back together?