Problems with Printer temperature

Hi,
So I have a Prusa Mendel I2, running gen 6 electronics with a J-head hot end. This printer has caused me so much stress and I have had many problems but I enjoy it, I have always been able to identify the problem and fix it however this time I am stumped.
the problem is as follows:
I am running Repetier host, with sprinter firmware, the printer connects and moves fine however when I tell the hot end to print to say 200 degrees, it will continues to rise to about 280 degrees it will then slowly fall back to 200 and it will then rise again. My first thought was a faulty thermistor however the temperature readings appear to be accurate.
As I said I am stumped, any ideas or suggestions would be greatly appreciated.
Regards
Pseudo