Volvo Repair: heating problems, air pockets, lengh

Question
i have a 96 , 940 2.3 turbo auto ,

have just had my radiator replaced , and have noticed that when i,m sitting in traffic the blow heating temperature is normal (nice and warm) , when i,m driving the temperature drops and the blow heating seems not to be working ,

could you tell me if this is a thermostat problem or something else , i ,m quite competant with minor repairs to car but not major problems , so would i be able to fix this myself .
any help would be greatly appeciated ,
thanks andy

ps , i just bought a tow bar off ebay , would you know the size of the bolts (width and lengh) that i need to bolt it on with ? ,

Answer
Hi,
  Sounds like you have a coolant problem...too little.
With cool engine check the level...sometimes air pockets form and problems occur..I doubt that you have a problem
with the thermostat..
      I don't know which size bolts you need.

                    Bilbo