Just did the re-solder trip to mine - and burnished the relay contacts with 600 grit. Every time a button is pushed (the 5 'middle' ones), it stresses the solder joints. Put on your squinters, grab a magnifying glass, and head outside - indirect light - bet you'll see hairline cracks in the switch solder points...

but since you're in there, might as well re-float all the solder.