I recently bought a 1999 Disco I which had a seized fan clutch. I replaced the clutch twice and it never stopped overheating. I then put the seized fan back on and it seems to cool fine. I also changed the thermostat and it didn't change anything. I switched to an electric fan and I thought everything was fine until today on a longer trip it started overheating again. It seems to be more present on highway rather than in town which I think is weird. Any idea what the problem could be?

 

Are both your electrical fans working, they should both come on when city driving and or when the a/c is on.

 

Yes all the fans are working.

 

Flush your cooling system, do you know how old your water pump is?

 

Sounds like the notorious radiator strikes again. D I radiators can become restricted quite easily. How many miles on your disco?

 

Was the replacement fan clutch that you used the correct one? If it spun in the wrong direction then that might;ve been your problem. If I remember correctly, the correct fan clutch screws on by turning it to the left.

 

The fan will spin in the correct direction no matter what fan clutch you put on, unless you install the fan on backwards, and yes, the engine fan on a LR is directional.
Since his problem was there before the clutch replacement he either has a bad water pump or a bad/clogged radiator.

 

There are 135,000 miles on the vehicle and as far as I know the water pump is original. It isn't leaking so what would go bad on the water pump? I assume the coolant is original too but it doesn't look bad. The electric fan I put on is an 18 inch Mark VIII with 4500+ CFM which I found are used in many performance upgrades due to its cooling power.

 

Original coolant...

How about starting with flushing the coolant system and see where that gets you.

 

The fact that it's happening at highway speeds pretty much tells you right there. It's not the fans. Your radiator is restricted. I had a similar issue about 5k ago. I installed one of these https://www.performanceradiator.com (I was in Phoenix at the time) and have had no issues since. IMHO I see no reason why it would be your water pump. Water pumps begin to fail based on bearings and seals leading to leakage. However, they'll keep pumping until they blow up.

You have a bad radiator.