Ariston combi boiler only works with a punch! something sticking?

My ariston combi boiler often doesn't fire up. However, with a swift fonzy like whack it does. The fan always comes on and it goes through all the motions but something is getting stuck. The hot water works mostly when the heating is on and turning the heating on works better when the weather is milder so something doesn't kick in properly when it's cold. I would think from this explanation someone who knows what they're talking about could work out what it is? Diverter switch? Any advice much appreciated, cheers.