Combi Gas Boiler?

My boiler is turned up to maximum on both domestic hot water and radiator temperature. The DHW is warm to medium hot at best and the burner continously 'modulates' giving warm then cold water (a bit annoying when in the shower!). The burner never appears to reach a 'high flame' condition. The radiator temperature also appears cool considering the maximum setting......Anyone got any recommendations please ? I will call in a boiler engineer to remedy but would like a 'heads up' before calling! The boiler is a Vaillant VCW GB242.....I think!