Octavia 1.9tdi starting problem

Skoda Octavia, 1.9 tdi 2008  45,000miles

When starting the car while hot, the engine sounds as if the engine fires in the wrong cylinder order, and the engine does not start.  After about 2 seconds of this dicordant sound, it sounds as if one cylinder fires before TDC and thus the engine stops turning abrubtly.  The engine will then start and run normally.  

This has happened 3 times.  After the second time, the engine would not start instantly as it would normally do, but had to be run on the starter for a few seconds.  After it started, it seemed to run ok.

I took it to my local Skoda garage who tested it and said the timing sensor had failed.  They replaced it and the car worked well for 3 weeks.  Then today, the engine started again as in paragraph 1, ie sounds like it has fired in the wrong order.  Can anybody help me with the diagnosis?  I have not yet taken it back to Skoda to see if the new timing sensor has failed.