my hot water runs cold and radiators don't always heat?

I have a baxi combi boiler about 4 years old. For a few weeks now hot water has on occasions gone ice cold and then takes a good 3-4 mins to heat back up. I rest the boiler by turning the dial to R which does seem to speed the process up slightly. I got up yesterday morning to find the heating had come on as timed but radiators were ice cold. When looking at the pressure gage it was between 0.5 and 1.0. i reset the boiler again and heating then kicked in. Since then the presure has fluctuated between 0.5 and 2.0. What do you think the problem is?