On Thu, 8 Sep 2016 18:36:23 +0100, Mike Tomlinson
wrote:

You seem to have the fault identified, but just to add another
perspective - since you're getting multiple failures of the solenoid
coils, and you're /assuming/ RoHS lead-free soldering is to blame
without having (yet) found evidence, perhaps you could look at other
common factors.

Fair comment.

Is the supply voltage to the coils stable, for instance? Voltage too high?

Without logging it (and I don't have access to a Dranetz any more) I
would venture that it is. Reading taken at random times show good
regulation, and nothing else in the place which is surge-prone
(electronics and incandescents) is dying at all.

Loose connection causing unstable voltage/surges/sags at the
coil?

Nope, snug as.