Whatever is causing water to be lost from the system is the problem, not necessarily the PRV.
Check the expansion vessel has not lost its gas charge, or has its diaphragm perforated. It should take several seconds with the filling tap open to recharge the system to 1.5bar (maybe about 10s). If it "recharges" very quickly this is an indication of expansion vessel failure, as is a large change in system pressure from cold to hot. If it goes over 3 bar the PRV is designed to "drip".
Look for underfloor leaks too.
Phil J, May 2008
