There are a number of things you could try:
- check the system is properly full of coolant, with no airlocks. I am not familiar with the Rover V8 but would guess there are recognised filling methods to avoid airlocks.
- check the rad is clear of debris: road oil and muck can significantly down grade its cooling capacity even though it might look OK.
- check your fan belt is properly tensioned, so the fan is cooling to max efficiency (if fitted with a mechanical fan).
- backflush the entire coolant system: rig up some pipes etc so you can run a hose into the system against the normal flow, leave it for a good 30 mins.
- try some coolant additive such as Water Wetter.
If no joy, some more expensive options:
new water pump
new alloy rad
new additional electric fan - Revotec or Kenlowe (or is this what you mean by 14 HD fan?).
ON another make I also know someone who fitted a small electric pump into the bottom hose, wired through a relay to a switch. He uses that to keep coolant flowing through the system when stationary. It's a pre-war car so prone to boiling.
