You really need to determine if it is lacking spark or fuel when it fails to start. If you don't have access to a fuel pressure gauge or spark tester, then after cranking the engine over for a few revolutions with a no-start, pull a few spark plugs and see if they are wet with fuel. If so, you are getting fuel and probably no spark.
If you have fuel but not spark, the Crankshaft Position Sensor (CKP) has been known to fail on the SHO V6. The CKP is not easy to change and while in there, you should consider changing the timing belt and water pump. Often the water pump will start to seep/weep, dripping on the CKP and causing it to fail over time.
If you have spark but not fuel, then the fuel pump is likely weak and failing.
-Rod