I have had the crankshaft position sensor cause similar concerns. Since you have had this problem come and go it could be your problem. Chevy has issued a TSB for the sensor here is a copy of it.
File In Section: 06 - Engine/Propulsion System
Bulletin No.: 00-06-04-014
Date: April, 2000
Parts are currently available from GMSPO.
For vehicles repaired under warranty, use the table.