Heater blows cold air on lows RPM's

[kennybhoy]

Hi, this is my 1st post on here although i am a regular visitor. I've bought my 1st X5 in December and over all been very happy with it.
Its a 2002 4.4 running on LPG. Ive had a few problems with it, mostly electrical and found most of my solutions on this site. (very impressed)
My latest problem has been. During this cold snap we've been having, my heater blows cold air unless the car is under pressure or travelling at a decent rate. No matter how long the car has been running, travelling slowly the car is freezing cold inside in no time at all. To get the interior warm again I've got to get back up to 60mph+. most of my driving is on the motorway so not generally a problem. But I've gotten now that I'm dreading coming into town coz I'm gonna freeze my knackers off.
Anyone? Any ideas? Please?