Problems with my Potterton Combi 80?

My Potterton Combi 80 boiler is having problems with the heating coming on, the hot water is still coming through but when the heating tries to come on it seems to light up the pilot for about 30 secs then go off this repeats all the time, if i try to put it on constant it seems to have a mind of its own, it does'nt come on straight away but when it does it does it's as above, coming on then going off, also when its on the radiators upstairs get hot but downstairs only get warm.