Rinnai infinity hot water service, goes hot and...

Question

Hi I have moved into a rental property and it has one of the rinnai hot water things now I have a control in the kitchen and one in the bathroom, i set the water to 38 and jump in the shower it is nice and toasty then after a minute or so it goes cold then warms up again then goes cold again constantly until I give up and get out totally annoyed. Neither the kitchen or the shower holds a consistent heat, thought I would see if anyone knew anything about these things as the estate agent seems to have no idea at all and I really want a nice hot shower! Thanks heaps!