My suprima 60L takes ages to heat the rads?

My Potterton Suprima 60L (installed 2001) on firing up Flames for only about 2/3 minutes then goes out and remains out for probably 5 minutes or more. Then on again for another 2 minutes and off for another 5 mins and so on, consequently it takes ages before the whole system gets hot. Everything else appears to be working OK. a thermistor has been replaced but to know avail