My c/heating hot water only gets hot IF you turn taps on /off couple of times.?

had 2 new divertor valves,had 3 engineers out ,because VAILLANT they cant do it ?if you want a shower you turn taps on / off 4-5 times then water v/hot .if you dont do this freezing cold/warm.yet boiler saying water 85 degs,boiler is in next room to b/room will adjusting the water pump make a diffrence ?