* Poway, when you turn the key does it crank ok and just not start, or does it not crank at all?
* If it doesn't crank, that takes your diagnosis in one direction. If it cranks fine but doesn't start, it takes you in another direction.
* Assuming it cranks fine, take a fuel pressure reading (you'll need the appropriate gauge, but the hookup on the M104 engine is very convenient). If you had no fuel pressure while it cranked, that would tell you the problem was in the fuel delivery. And you could decide on your next step down that path.
* If you have fuel pressure, you ought to check for spark next. I believe your engine has coil packs (rather than a distributor cap and rotor arrangement). You may need to remove the top, center panel on the motor, exposing the plug wires, etc. (M104). Some sort of inductive current probe is handy here to check a plug wire while someone cranks the engine (you can also do it alone using a remote starter). If there's both spark and fuel pressure, the next step is different than if there's no spark.
* To back up for a moment, if you have a scan tool it wouldn't hurt to scan for faults in the engine management systems before doing anything else (and you don't have to wait till the engine won't start to do the scan).
* I assume that you have a small window of opportunity when this failure happens. So it may mean being rehearsed and carrying a few tools in the car. The times I have seen this occational failure-to-start-hot problem, the engine also died while running and would then crank fine but not restart until it cooled down. Then it would start like nothing had ever happened. In that case the crank position sensor was defective (and there was no spark). You've got several diagnostic directions possible here, so you need to start by getting some of the basic data. Good luck.