Grant oil boiler

I have been today to a new boiler installation...the boiler fires and heats up the rads/hot water cyl,all rads have trv,s...problem starts when the flow and return temp start to approach the boiler thermostat setting,the boiler switches off and 30 secs later switches back on.the installer has had grant out to the boiler who have replaced the thermostat and pcb...no change whatsoever !!,i never figured there would be,i remember reading about a plumber with a similar problem,anyone had this before? Considering a plate heat exchanger or buffer to give me a chance at the boiler condensing but i remember that someone had this before and it was a turbulence issue around the thermostat phial...Grant have been no use at all

 

its a combi 26 kw,flow temp stat been turned down even at 60 it still cycles extremely quickly.i have been called in by client as boiler only been installed couple of months ago and it is gobbling oil,done my own combustion tests,oil pump pressures etc as have grants own tech...all to spec,seen this problem several times now ,know if i installed a good non condensing boiler it would cycle correctly and burn less oil.pump is set on no. 3,wilo pump as to latest boilers. Thanks for replying