[David Hodgson]

Dave, I had a 6 cyl crossflow Cortina once that had the same issue as you describe. Temp fine under 110 k's but sit on more than that and temp crept up.

I never solved the problem untill the block was caustic dipped when I had it out for a work over.
Bit difficult with an alloy head to flush it with caustic. Maybe there is another good product available for flushing iron blocks without harming the alloy bits.