Baxi Combi Boiler?

I have had an engineer out to my central heating system and he has said that their is a blockage in the system and I would hve to do it my self because it does not come under the insurance and iwould have to pay ,any ideas how i can put it right.
the radiators come on when they feel like it and the shower is hot and cold all the while , the engineer said the boiler is ok.
Can anybody help me

Vince