Combi Pressure too high!!?

Hi there. I have a Halstead 30/90 combi boiler which, since I replaced the diaphragm in the diverter valve, is suffering from high pressure issues. 3.5 - 4.0 bar to be precise. Now theres no big bangs or knocks. In fact the boiler works perfectly. I have replaced the relief valve, checked the air pressure in the pressure vessel and checked the loop feed. All is fine here. The gauge seems to read correct. The relief valve outlet will drip at 3.5 bar. I periodically release the pressure through one of the drains on a radiator. What I have noticed though is if the pressure falls below 3 bar it kills the heat in one of the radiators. Another point to mention is I took the seals out of a couple of the thermostatic valves because the radiators weren't getting hot. Any feedback would be much appreciated. Thanks.