The possibility also exists that the Legacy 600 transponder "failure into STBY mode" may be one of the early manifestations of the increased incidences (and consequences) of lead-free solder (imposed by the EU's RoHS Directive)
see
link and
link
and
link
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Problems noted thus far are:
a. Tin whiskering (hair-like protuberances that grow over time from-lead-free solder that are capable of shorting out surface-mount components on circuit-boards or introducing intermittencies that can be frustratingly "once off" due to individual whiskers completing a circuit but then melting or recoiling under the current flow, or possibly as a result of the changed magnetic field). The requirement to be lead-free has caused the percentage of tin in some solders to exceed 60%.
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b. Brittle fracture surfaces in lead-free solders causing intermittencies (a greater incidence with some of the new formulations of lead-free solder - of which there are about 700 variants).
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If the two Legacy pilots want their "get out of jail free" cards stamped, they'd do well to acquire some expert testimony on the new failure modes in PFD's and other sensitive transponder type avionics that are slowly being introduced as the avionics industry compulsorily transitions across to lead-free solder - with its attendant new suite of intermittency problems.
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When considering the performance variations between pb-rich and pb-free solder, think the difference between a glue that has had it's binding agent removed and replaced by greater amounts of gelatin. Alternatively think of what super-glue used to be like when it first came out and how uselessly ineffectual the modern denatured super-glue is nowadays (after the safety wowsers forced its manufacturers to make it less lethally effective).
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You can read an extensive array of articles on "tin whiskers" and the problems it causes in modern electronics (since the introduction of lead-free solder) at this
link