I hope you guys can help me with this

When my car starts the temp needle rises quite quickly from cold to midway on the gauge and there it'll stay quite happily, infact I've only ever known to rise higher than midway on a couple of occasions and it wasnt by much. 

Even though I've had the car for 4 months now I've never been caught in  traffic heavy enough to cause a prolonged standstill until yesterday - after about 5-6 mins the needle started to creep up until it was almost in the red (this caused the engine to sound slightly ruff, like it was pinking).  Once I was in clear traffic again the temp dropped back down to normal.

Before I bought the car the radiator and thermostat were replaced but until yesterday I've had no reason to suspect that there could be a fault with them. The fan is working ok and the themostat hasnt required topping up since i got the car, so I know water isnt leaking through head gasket ( or I hope!).

With the cars being so well engineered i cant believe this is normal, any advice please.....

thanks

West

e34 525 sport