Before flushing, remove thermostat for full flow. Then REVERSE FLUSH the whole system inc Rad, or separately, but reverse F~ both the RAD and the Engine.  Before re-fitting Thermostat, bench test it whether it's the old one or a new one, and ensure it opens and closes correctly as per instructions on a reputable "Engine Service" website. 
 
Even tho I dont think this is your problem, theres no harm in ensuring a clean Cooling System.
 
If you know the history of your heads, and you know theyve never been over-heated, then you're very unlikely to have hairline cracks in oem Cast Iron Essex Heads.

in rod we trust

gtv6capri
wow thats expensive flush.  Just take the radiator out and drop it into a Natrad or local repair shop for a dunk in the tank and a pressure flush, also make sure you have a thermostat fitted

I rag natrad and that quoted me 800 + for a recore so I would imagine that a dunk and clean would cast about half that...   a new 2-3 core would cost 250 in alloy..  it is expensive rad cleaner but seem to be th best one on the market that works ,  it flushes the block and rad at the same time , no real use in doing a rad change unless I try remove as much scale out of the block as well so I don't end up with another block rad ..  other than that I was going to pull the heads off to convert to unleaded valve seats this yr and new valve stem seals as well , so ill just have a good think on when or what stage to do it..
 
I took the capri out yesterday just local drive and kept a eye on the temp didn't get to half way as it was bit cooler yesterday but had thermos fan on , it didn't stall at all .. but I know if it reached half way it would of stalled and ran like crap , missing and stalling . its weird alright as I remember when I put new head gaskets on one looked dodgy I put them on and 4 months later head gasket blew and in the spot I thought was dodgy payen gasket.  it heated up way over half lucky I was only 2 min from home but the car didn't carry on like it is now at half way .. it stalled few times when it was blown in the 2 min to get home ..  the only thing I can think is just maybe the temp switch being as they are is showing less than what it really is and at half way its actually running very hot , but I have no way coming out of over flow tube .. so I think I try the flush and get new alloy rad as I can find any that resemble oem one 

HEY ROD !
 
The only thing you havent changed is your IGNITION SWITCH &/or COIL POSITIVE IGNITION WIRE/ballast resistor ........ !
 
ASSUMING POINTS ARE STILL IN, DO THE FOLLOWING which takes 5 minutes, and bypasses all above !  .........
 (if electronic, youll need to check with maker on safe method to "hotwire" the ignition)
 
Get a regular piece of auto insulated wire with Aligator Clips at each end.
Place one Clip on POSITIVE or IGN side of Coil ...........(NEVER on neg Points side),
connect other Clip to POSITIVE BATTERY TERMINAL. 
 
After youre sure "jumper wire" is secure, start the engine and go for a roadtest. If it continues to behave, ALWAYS disconnect the jumper wire when you're no longer running the engine and/or parked the car..... Re-connect and disconnect each time you use the car, until youre convinced theres no more poblems. 
If this proves the problem, start with looking for a "broken wire" or loose connection in the Coil to Ign Sw Wire before replacing the more complicated job of Ignition Switch, assuming you havent got loose connections at the actual switch itself.
 
nb...........  when you go to turn off engine at key, it will likely keep running until you disconnect "aligator clip" at Coil FIRST 

Can 100% Guarantee you it wont be IM Gasket .............  try my suggestion as above as it only takes 5 seconds
 
This way, (starting at simple things first), you wont feel so bad after many hours of un-necessary work that gives you zero results
 
Hey Rod, it's not only the resistor you're bypassing; it's bypassing ALL your ignition wiring AND Ignition Switch which have potential for MANY partial breaks in the wires at ANY point, and poor/loose/worn-out IGn Sw Contacts !!!!!!!!!!!

Have you tried the "hotwire" 5 minute test yet (Pos jumper wire from BATT POS to COIL POS) 
 
Never Bat Pos to Neg points side or you'll have melt-down damage immediately! 

Im in suspenders waiting for this "hotwire" test ............. a 1 minute job that tells a lot
 
Have you done it yet ?

Perfect time to try "hotwire" 1 minute test was when you had points and condenser installed, but..........
 
If your electronic system has only one wire to POS side of elec coil, you can still perform same hotwire test
 
It appears you're making a mountain out of a grain of sand .......... 
                   I would feel extremely unhappy if I ignored the suggestion of a Qualified Mechanic and went thru
what you're going thru and then found the "hotwire" test proved your problem was a simple POS wire from coil to
Ign Sw , or BAT pos wire to IGN Sw, with the worst scenario being a crook Ign Sw !!!!!!!!!!!!!!!!

Regulated voltage is 14.4v so if you bypass the resistor for too long you risk damaging the coil and other ign system components.  Even with the resistor electrical fires and melted ign switch connectors were the reason many people started fitting relays in their elect system.

hink this one is about the closest one I can find that should be able to mod and mount up