Intermittent Starter Problems
 

I have a 2001 Audi A6 Quattro and have been having problems starting it recently. I started having problems the other day where the engine would crank but it won't turn over. The battery is fine and the tank is 3/4 full. After a couple of tries it started and I put some octane boost in because I thought it was bad gas or water in the tank. Then two days later it happened again except I couldn't get it started this time and had it towed to a mechanic. The mechanic hasn't been able to figure out what the problem is yet so I was wondering if anyone has any idea why this is happening. The car has a good number of miles on it (around 170k I think?) and the cat on the driver's side was replaced recently. I thought I've read about some electrical problems/software that could be causing this and if this is the case how would I/te mechanic figure that out? Any advice would be greatly appreciated!!!