Our PCM failed twice.
1st time car had been parked in garage for a couple of weeks, switched on ignition, PCM screen came up with Porsche logo, then a relay click, black screen, relay click, Porsche logo - repeated over and over. Clicks were every 8 or 9 seconds. None of its functions worked, and the 'trip' features were dashed out (no data) on the bottom of the speedo (i.e. overall mpg), presumably fed by the PCM. Diagnosed as 'failed', new unit fitted under OPC warranty.
2nd time - 3 months later car and been garaged for about 2 weeks, set off from home, half a mile later stopped for fuel, on starting up again black PCM screen, the rhythmic clicking as above, PCM totally dead, no Porsche logo this time. Unit again 'failed' and replaced.
On 2nd occasion it actually flattened the battery over 2 days waiting to go to OPC as it was apparently continuously running/clicking, even with car switched off and locked. Technician even commented on how hot it got!!
When querying why it had failed twice in 3 months, i was told by the OPC that the failed units are sent back to Porsche, they examine them and repair/refurbish if possible, and these are units that are then sent out and fitted when one is replaced, so second failure may have been due to something to do with a repair, rather than our car causing failures.
So whilst ours were total failures, no intermittency beforehand, if your period of black screen and not working is around 8/9 seconds and there's a relay click, that fits with one of our symptoms. Not sure if this helps but.......
