Hmm
sounds very similar to my old girls problem.
new rad,with baffle,
new viscous and fitted dataleks air con mods so i could drag the fans in.
Happened once fitted s/h head ,new thick quality gasket,new bolts new water pump ,bled it . Lasted 300 miles
fitted a second hand head ...again had it tested all good,new gasket ,new bolts,new thermostat. . stayed cool for over 100o miles but always at 12 o'clock my other rangey stays at 11 pulling a horsebox with 60 sheep on board all ok.
1000 miles later overheating stays at 12 then whoomph in the red . Let her cool,no water loss released the pressure in the morning. Started her after she gets warm all hoses are hard but not bursting hard so to speak.
Gave up in the end ,fitted second hand engine £500 with all ancillaries with 3 month warranty . Cheaper than the head gasket ,bolts extra time etc . Swapped over she now sits at 11 oclock and i can pull the box with sheep without having to watch the temp gauge all the time . Sold the injection pump,turbo and a few othe rbits and bobs for 300 pounds .
I know everyone says its the heads etc but im beginning to wonder if its something to do with either the block or perhaps clearences. One knowlwdgable ole lad told me id never do any good unless the block and head were perfectly machined and i should have used a cheaper head gasket as they allow better compression and hence sealing . Dont know,cant comment but i gave up in the end and did the swap.
I think i changed the hg correctly as ive changed caterpillar ,perkins,ford,vw audi head gaskets and had no problem afterwards.
On another matter ,i was looking at putting a more conventional rad into her before the swap,i.e in at the top out at the bottom from say a disco ,td5 etc,has anyone done this ?
reminds me of my old stag which eventually i put an electric water pump in in the bottom hose and blanked off the original water pump. Oops showing my age now but to all you youngsters ,theres nothing like the burble off a triumph v8 ,well as long as its looked after that is.
 
Maybe needs the block deck machined.

My V8 would pull a horse trailer with 2 shires in it, no sweat full thottle in areas.

The highest was on a steady grade, foot to the carpet going round a Truck, yer you can overtake loaded with a V8 ;)

Temp went to 1/2 usually 11 o clock ;) fan howling, nothing broke though ;)

These damn dervs ;)
 
Update, rad water pump and stat a off, pump and stat ok but I have doubts about rad. i cannot drop a 6mm nut through either way but if I lean rad @45% upside down and add water, it is coming out of the other pipe in seconds leading me to think that there is no seal between the halves. is there supposed to be ?
 
Not sure, think it’s just the baffle at the top but I’ve never actually cut one open.
Wammers will know.

No signs of it leaking anywhere? Some have bung underneath
Any idea what brand it is. Most go for nissens. I had an expensive one at first, then pump blew soon after I fitted and smashed rad out, replaced with cheap one (b******t I think).
 
Update, rad water pump and stat a off, pump and stat ok but I have doubts about rad. i cannot drop a 6mm nut through either way but if I lean rad @45% upside down and add water, it is coming out of the other pipe in seconds leading me to think that there is no seal between the halves. is there supposed to be ?

If you cannot drop a marble or the nut through the baffle would seem to be present. They are not sealed as such they just sit in a slot in the header tank and will allow a little to run past them but prevent major free flow, which is what they are there for. Does your heater get hot? You may like to back flush the matrix with a hose whilst you are at it. Before stat opens coolant circulates from water pump through matrix and into head via C pipe. If that circuit is restricted it can form air locks.
 

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