Cold Air blows above 50mph

[JDF]

12 months ago I bought my first X5 2004 4.4i. 112,000 miles. Being a massive fan of the V8 I decided I would get her back to as good condition as I could. I've spent a small fortune (twice what I paid for the car which was £3500)(I know, don't go there - could have bought a much newer one!)

There are still some issues but the most immediate one is a heating one. I run the car with aircon on pretty much all the time - hot or cold weather. A couple of months ago I noticed on a cooler day there was no heating available. I took it to the garage and with some investigation they found a small water pump leak but also changed the heater control valve (only available from BMW and costing £200!) which regulated heating getting inside the car. They also replaced the thermostat.

Since then - heat is available and lots of it but only around town at slow speeds.

If I go above 50mpg for most that 5 mins or so the heating goes cold. If I stop or slow down to 20mph - it comes back on. Fan continues fine - it's just the temperature of the heating.

Its a local garage and not a BMW dealer I go to - I tried BMW when I bought it and they spent £4000 for me and took 9 weeks in their very posh garage (it only went in for a service.).

This cant be a unique problem - can someone tell me what the problem is from these facts please?