Land Rover Discovery II 2000 with heater on it blows cold air at low speed - at 50 or more it blows warm/hot air - as soon as you stop or slow down it again blows cold air? Any idea what it might be?

Hi welcome main problems are the abs valves causing the 3 dash lights (search 3 amigos online) a really badly sited compressor can cause air suspension models to fail giving the bags a slow leak the V8 is a solid engine but can have radiator expansion tank and cap failures
the other issue is electrics windows and door looms are problematic

What blows cold air? The heater? Sounds like a radiator problem. I'd have someone flush the cooling system if it has not already been done. Could also be a bad or going bad Coolant pump, unless it has been replaced. It might be smart to do both at the same time. If the coolant pump is bad, it could explain no circulation of hot coolant through your heater coil.

The ignition system is controlled by the ECM. Check power at fuse box in on right side under dash. Check fuses right side engine compartmentlower dash panel if all OK check power to ECM located right side "A" pillar in alloy box. this unit could be faulty if no spark at coils, assuming they have been connected correctly. may have to get tested for codes at L/R dealer or someone with test tool. Another source of problem may well be flat battery in transponder key. The indicator light on the dash willflash rapidly for 10 seconds if the vehicle has been unlocked with the remote and the drivers door is opened if the battery in remote is getting low

As per your problem mentioned.There are few solved help links to troubleshoot these problems.Go through the list and Click the link below:----
Air conditioner blows air but not cold air http://repairhelpcenter.blogspot.in/2012/07/air-conditioner-blows-air-but-not-cold.html

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Car overheats and Air blows hot at idle? http://howtobyme.blogspot.com/2011/11/car-overheats-and-air-blows-hot-at-idle.html

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Heater blows cold? http://schematicsdiagram.blogspot.in/2012/02/heater-blows-cold.html

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Heat blows cold on drivers side and hot on passenger side? http://schematicsdiagram.blogspot.com/2011/12/heat-blows-cold-on-drivers-side-and-hot.html---------- Hot air on the driver's side vents only? http://repairhelpcenter.blogspot.com/2012/01/hot-air-on-drivers-side-vents-only.html

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2005 Chrysler PT Cruiser: A/C Blows Warm? http://whoisbyme.blogspot.in/2012/06/2005-chrysler-pt-cruiser-ac-blows-warm.html

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1992 Toyota Camry: A/C Control light Flashing and A/C not Blowing Cold? http://technoanswers.blogspot.in/2012/04/1992-toyota-camry-ac-control-light.html

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No heat, blower blows only cold? http://technoanswers.blogspot.in/2012/02/no-heatblower-blows-only-cold.html

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These will help.
Thanks.

Sounds like heater blocked up.

when engien is cold ,top up coolant bottle with antifreeze and see what happens ,or the heater hoses are on in reverse

Check your coolant level. Does it overheat at all?

oil pressure dangerously low so overheating and bearing failure inimimently soon

check ur thermostat its not opening to let heat pass through

i would check the thermostat first to see if stuck then check heater core