HIGHER THAN NORMAL PRESSURE ??

My boiler sits happily at 1 bar when cold and 2 - 2.5 when fully running. In the past couple of weeks this goes to 3 bar + and can only be lowered by reducing temp. Ive checked the expansion vessel and topped it up with air.... Any further ideas ???
Secondly, how do u test that the PRV is working - could this be sticking or broken ??? Thankyou