how can i fix my baxi combi instant 105e?

my baxi combi instant 105e keeps loosing water pressure every 1-3 days, i keep topping it up and it will go back down, also downstairs there is sometimes no hot water coming out the taps for a while (but all other taps upstairs are ok) and the water tempreture keeps dropping down.
also i went into loft to find a small cistern which was empty and dirty but still connected up, the byelaw kit was half there but no service valve? can anyone help please