Problems with my central heating?

Hi im hoping someone can help me with a problem im having with my central heating. first of all i have got a combi boiler which i have had for 3 years now, the problems iv just started having are. When i come to use the water its taking long to warm up. And also my radiators are taking ages to warm up. I Have brained my system the other week, then put some cleaner in there for three weeks after three weeks iv then brained the system again, the water was abit dirty but mainly clear, I have bleed all radiators, all upstairs radiator flow valves are a half a ture open which dose help a little bit with heating up the house but is still taking ages to warm up! Has anyone got any ideas what might be the problem! I have yet to have the boiler serviced which i am looking into getting sorted soon but will a service on my boiler sort these problems i am having? please help thanks