I have a 1999 Blazer that has an intermittent stall and intermittent no start.There appears to be adequate fuel pressure and spark at all plugs.The check engine light is on with P0420(catalyst efficiency)in memory.If anyone has any idea's I would really appreciate hearing what you think.

What is the fuel pressure and leakdown rate?

The P0420 may or may not be a problem. Reset the ECM with a scan tool and see if the SES light comes back on after driving.