pgmartin:
12V come directly from a commercial switching power source with 2.5 amps capacity (Can this be blamed?).
Measure the voltage under load...?
pgmartin:
Each led module consumes 40mA, and I have 42 modules.
Five LEDs could be arranged as a string of 2 and a string of 3 in parallel to work from 12V. 20mA for each string adds up to 40mA. That's just a guess though.
Do the dead LEDs form any kind of pattern in the modules? Do they die in similar groups of two or three LEDs?