TNY279 going into continuous mode operation

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We currently use a TNY229 for a 15W LED driver and are finding that it randomly alternates from discontinuous to continuous mode every few cycles causing the device dissipation to increase significantly from an original 60deg.C to over 100deg.C
We didn't notice this on our prototypes.
We have tried re-tracking the pcb to the ideal as per the application notes but this doesn't help.
The original devices came from the UK and these were supplied from our China factory.

Any clues.