One more thing: after doing more testing at the request of the Bumblebee guys, I could see that IOMMU kernel configuration has an impact too. Without this option compiled in:
Support for hardware IOMMUs in IBM's xSeries x366 and x460
systems. Needed to run systems with more than 3GB of memory
properly with 32-bit PCI devices that do not support DAC
(Double Address Cycle). Calgary also supports bus level
isolation, where all DMAs pass through the IOMMU. This
prevents them from going anywhere except their intended
destination. This catches hard-to-find kernel bugs and
mis-behaving drivers and devices that do not use the DMA-API
properly to set up their DMA buffers. The IOMMU can be
turned off at boot time with the iommu=off parameter.
Normally the kernel will make the right choice by itself.
If unsure, say Y.
while I would not get "has fallen off the bus" I do get the following messages (rminitcontext etc) and the card is unusable.