General knowledge engine question.

[boxhead]

Ok, I realise this is not Cobra related but there is a fair bit of knowledge in here so I thought I would ask.

My wifes car (1994 Ford Capri, twin cam, non turbo) has had an on going problem for a couple of months now.

When driving at 60 kph the car is fine, all ok
But get upto 80 or more and the temperature creeps up, it will go all the way to hot if you keep going at speed.
Before Christmas it developed a coolant leak in the bypass air solenoid (this allows crankcase fumes to be drawn into the intake manifold once the car is upto temperature) I replaced this with a decent second hand one (new is no longer available) I reassembled the car but discovered further coolant leaks in both the water pump and the radiator (no inhibitor), so I replaced both.
Now I am not 100% sure if the problem started at this point or was ongoing from when it had coolant leaks.
I then replaced the thermostat and the radiator cap. Still no good.
I then checked camhaft timing (as timing belt is removed to replace water pump - I also replaced the belt while I was in there) and the timming was fine.
Next I thought it may be a faulty solenoid I initially replaced, so I clamped off the air hose, still no good.
I was then scratching my head as it did not present itself as a head gasket issue, but thought that there is nothing else left.
So 3 weeks ago I pulled the head (could not see evidence of head gasket failure, nor a running lean or burnt valve issue), sent it to a machine shop for pressure testing, resurface and replace valve stem seals ( I do not have a valve spring compressor)
Well after being promised I would have the head by the following Friday, it ended up taking him 3 weeks (he lost a valve collet), I got the head back this morning, I cleaned everything and refitted it, only to have the same fault still occuring.
Really clutching at straws now I thought that maybe the new thermostat is faulty, so I cut the centre out of my old one and fitted it, still doing exactly the same thing.
The water pump appears to be flowing correctly, as I can see the water passing over the radiator tanks inside fairly quickly with thermostat removed.

And that is where I am at now, so does anyone have any ideas?