Doddy, if the stall is caused by the load required to cut into the workpiece, it would only cause a shallower DOC than expected, this would not cause the tool to eventually spin in free air, because each cut layer adds a new DOC in. Once it has stalled on plunge enough times to equal the DOC, there will be no load to cause it to stall.

I say check the wiring, because it happened to me (and it was a bugger to find!).