one cold radaitor?

i have been having a problem with loss of pressure in vokera mynute16e release valve was opening pressure keept rising to 3bar.had to keep topping it up after heating had stopped .i checked for pressure in expansion vessle which was there but was gone in a few seconds no water came out.i pumped vessle up again but needle on boiler gague started to go up!!i released some pressure out of vessle topped up filler to 1.5 bar .boiler appears to work comes on does not loose pressure all ok but i have one cold radaitor.we only have 3 rad downstairs cannot be sure but i feel this must be a different fault. any advice would be great.