2007 Skoda Octavia cold heater

Hi all,a mate of mine has a 2007 skoda octavia 1.9 tdi ambiente,The problem started a few months ago thus far he has spent over £1100 yes £1100 trying to solve it to no evail,i will describe the problem then list parts and repairs so far.The fault is for the most part the heater blows cold air at normal engine tempreture,it sometimes when it wants blows hot air as it should.What it does though wich no mechanic including the main agents can explain is that when the car is STATIONARY the engine up to normal running temp the heater will blow hot air all day long even reved to 3000 rpm for up to an hour the heater blasts hot air out,now for the strange bit with the heater blasting out hot air if you drive the car in less than a mile it will blow from hot to cold and stay there blowing cold air no matter how far you drive or how fast,it does this each and every time he drives the car CAN ANYONE HELP PLEASE,the iobs done and parts are as follows
Head removed skimmed,pressure tested
New head gasget
New water pump
New heater matrix
New heater facia controls
New thearmostat x3
New heater and various hoses
New expansion tank
The system has been pressure tested twice (ok)
There is no coolant entering the engine or exhaust systen
Has been bled of air several times
All intenal sensors tested working fine.