air conditioning fault

I have a strange fault on my air conditioning. If it's on lowest setting 6 times out of 10

cold air will come out of all the vents-2 driver, 2 passenger. However, sometimes 3 out

of 10, I will get blazing heat out of the drivers, yet cold out of the passengers. Similarly

if I change the fan level or temperature, it will usually start sending heat out of drivers, cold

out of passengers. This usually carries on for 20-30 minutes when it will revert to cold out of

all four. If I stop the car for 5 minutes, it will revert to cold from all vents. On 2 occasions, it

stayed at hot out of drivers, cold on passengers for an entire 3 hour journey, in baking sun (35 degrees)

which was very uncomfortable.

Anyone else had this problem?