Protherm 80 e intermittant hot water?

Since turning timer off but leaving boiler on winter setting I now find that boiler only fires up 1 in 2 times when I turn on hot water. Stangely , it may work straight away at 6.00 am but not at 9.00am and vice versa. Driving me mad. Discovered that if I put heating on, run tap slowly water is hottish. I am afraid if I book an apointment for engineer to call, symptons may not be there and I am still lumbered with a call out fee. Any ideas please?