Boiler cutting ouit?

We have recently had a new central heating system fitted, and are having problems, what we have is the boiler when set to run comes on for 15 mins then goes off for about 5 mins this happens if we set for hot water or central heating.the boiler is a Ideal icos HE18
a type HSA3 Danfoss controller,a Danfoss FP/CP 715-S1 programmer and it is wired to No1 A fully pumped system 3-port mid position valve (Danfoss)
The thermostat as been taken out with just a switch for on or off (so we know this is not the problem)
The Ideal boiler rep came last week and said the that was ok he even changed the PCB (for peace of mind)He thought it could be the Progammer or the 3-way valve
could you please help in any way.