Hot Water intermittent and radiators only warm..Vaillant Turbomax Pus 828E?

I have a 828e Turbomax Vaillant boiler. The temperature on it keeps dropping constantly and then the boiler kicks in and increases the temperature and then the temperature after reaching a certain point, starts dropping again but the boiler does not kick in and the temp remains low e.g., 22 C. Then when I restart the boiler, the same cycle repeats. Also the radiators upstairs are only luke warm and not hot. Downstairs ones remain cold, although bled. System is only 4 years old. Pressure on boiler keeps dropping as well after I turned a grey valve at the back of the boiler on the right. But when the hot water tap is opened, the boiler ignites and the temp. starts increasing but the water is hot then cold, then hot then cold. (Water in the Kitchen is good HOT) Please Help!!!
How can I maintain the pressure and temperature of the boiler. In other words how can I reverse the effect turning the grey valve has had (Pressure keeps dropping). Before the pressure remained constant.