Both heaters blowing cold intermittently and overheating under load can be caused by air locks in the cooling system, this could simply be the coolant level has gone below the minimum mark.
Under normal conditions the coolant level should not alter and should stay within the minimum and maximum mark, if you are topping up on a regular basis then you have a leak or is being blown out through the cap.
It's possible you may have early signs of head gasket failure but before pulling the cylinder off check out the following items first.
Firstly run the engine with the cap removed, now observe the coolant in the bottle you should be able to see coolant returning to the bottle via the return hose, if you cannot see this happening then remove the return pipe and check the restrictor inside the pipe is not blocked.
If you are sure the restriction is clear but still no coolant returning then read on
Early MK2's had a problem with the water pump impeller which may not show itself as a problem until the car has to work harder say on a motorway or towing.
Check engine water pump for detached/sheared impeller, you can do this by removing the thermostat and housing,the impeller is now visible through the opening, the impeller must be firmly fixed to the pump shaft and no signs of breakage or looseness.
There is also an electric coolant pump to aid circulation through the rear heater and booster heater, this runs all the time the engine is running and for three minutes after switch of (115ps AUY engine only) this is to prevent heat build up after switching off engine.
If you cannot hear it running place a hand on it, you should feel a vibration from it.
If you can nether hear or feel it then it's probably not working.
This pump is located on the bulkhead behind the fuel filter, see additional
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