.
That sounds frustrating! I have never stacked 7135s but I would have the same thermal concerns as you.
But I think many people have done it so maybe they have some tips on how to increase reliability. Maybe the driver is not dissipating heat well enough to the pill?
Sounds very frustrating rayfish. 
Mountain Electronics has quality 7135 chips at good prices: AMC7135 / MOSFET
The LEDs or 7135s ?
I hate soldering those darn Chips — I’d add 4-8 chips on a driver — Always have at least one not connected or faulty — That’s why Richard charges so much to add them