Discharging boiler pressure relief valve

garbid

The pressure relief valve on my Burnham boiler (for a gas-fired hot water home heating system) has begun to discharge slightly - not full force by any means, but a heavy drip - each time the boiler temp and pressure reach 200 degrees/35psi, shortly before the furnace cuts off, at which point the discharge stops until the next cycle.  I drained and recharged the expansion tank routinely two months ago as I do once each heating year (there was no discharging from the PRV at that time) and the Watts pressure relief valve was new about 5 years ago.  No modifications have been made to any settings on the heating system or boiler and I've never experienced this discharge before; it only started about 3 weeks ago.  Any suggestions as to what might be the cause?  Thanks.