I was having an issue with my GD i30 occasionally being hard to start (turns over a lot of times before struggling to start), sometimes tacho not working when difficult to start. Also stalled (auto) a few times when stopping at stop lights etc. This was happening once or twice a week.
I ordered a replacement Crankshaft Position Sensor (about $150Aud) and installed it myself, and have not had any issues for about a month so far.
Installation was easy - Jack up the car, and from underneath the car you can access the Crankshaft position sensor, and undo the 10mm bolt securing it for the front of the motor (Make sure you remove the O-ring if it stays in the block) Then its as simple as undoing the electrical plug and replacing with the new one.
Worth a go for people struggling with this issue.