intermittant hot water, shower?

Hi have a rsf 280. for some time the showers has not been getting hot water boiler comes on 10-15 second then goes off. sometimes depending on where the shower knob is stays on but never hot, taps are fine no problem, plenty of hot water. 2 showeres both the same? The gauge on the front reads .4 off .45 on, hot water on tap or shower?