pressure increases when switched on?

oh dear
roughly speaking -- a system set at one bar cold should rise to about 1.5 bar when hot
now two things
first
first if the system has 101 radiators then the vessel in the boiler may not be big enough to accomodate the increase in water volume when heated.
second
Lets assume you have an average property and it still goes to 3 bar - I would suggest the pressure vessel is faulty - that is it is full of water instead of air (nitrogen)
you may get away with recharging it but if it is faulty then the pressure (in the vessel will fail) and the system pressure again will rise to 3 bar.
You can recharge if you can find nipple on the vessel at about 0.5 bar but you have to make sure system pressure is reduced to zero first- this does not mean you have to completely drain the system of water

as for boiler cycling rapidly - check temperature of outlet pipe --is it very hot when boilers shuts down (after one minute)- if so then you are short of water circulation -- make sure pump is on full speed (if adjustable ) make sure at least one radiator is always open.

best I can do for you my friend without being there

good luck

dave broadbent
June 2007