Water Pressure Gauge rises too high on my combi?

Dear Friend
Well I hope you are a friend because I need some advice, the pressure gauge on my combi boiler just increases above the initial setting.
I have tried to fault find myself but am lost. I bleed the system of air after putting on excess pressure, bleeding every radiator in turn, I usually start downstairs and then go upstairs continually ensuring the pressure is slightly higher than what I require by topping up, but finishing with the pressure at 1.5 bar. When I turn the heating on the pressure rises to about 2.8 bar and then when the system cools down it will have dropped to below 1 bar. I have been through this exercise repeatedly with the same conclusion every time so that now I have to top up every time I want the heating on and then I have to monitor the boiler so that the pressure does not go to high. Sometimes this happens and I have to release pressure or turn the heating off all together.
I am out of work and cannot afford to get anyone in, so I need some guidance, or do I have to wait to afford a specialist. I would appreciate any advice available and any explanation as to why this is happening. The boiler is a Ravenheat RSF 820/20T Fanned combi

Thanks Tommy Myles