halstead finest gold tripping out ?

i have a problem that affects hot water and heating . if i try to run a bath the boiler runs for a few minutes then turns off ,as the water temp drops it fires up again , it keeps doing this untill the bath is run or it deciedes its had enough and trips out . heating is the a similar story , the boiler fires up ,rads get warm boiler switches off , this happens a few times then the boiler trips out .
1 thing i noticed is the burner flame never gets smaller it just switches on and off , i suspect the modulating gas valve is not working properly but as its a very expensive bit to replace id rather know if thats the problem before i get someone to replace it . any one had this problem ?