Vaillant combi, Honeywell CM927, Hot water issue

Hi All
I have recently installed a Honeywell CM927 to control my Vaillant Combi boiler. Few hours later my wife screamed from the bathroom as no hot water in the shower. After switching off & on the boiler, later on everything worked fine. A couple of days later, the same thing happened. No hot water and the boiler was playing silly. The boss (wife) did not want anything to do with controller. When I called a plumber he said this could be due to air in the radiators. I had to disconnect the controller until I understand what causes the hot water to switch off.
Is the problem caused by the boiler or the controller? and how to troubleshoot?
Thanks
Bashir

Your cm 927 controler do,es not control the hot water side of your boiler.

we require more detail of the boiler ie; which model you have, to give any sort of diagnoes

Hi Bob
Oh sorry, the model is 828E.
One more detail I noticed is that the pressure keeps going down closer to 1, after a day or two and not sure why. Could this cause this problem?
Many thanks
Rgrds
Bashir