Perhaps faulty spares ?
Assuming they are the mineral insulated type, could they have been stored in damp conditions and have taken in moisture ?
I've had a bad experience with "damp" cooker elements.
I believe that moisture distributed within the insulation is driven to the cooler ends as they heat up, and a quite respectable cold insulation resistance can fall dramatically as the moisture concentrates.
Try an insulation test after heating up.
