Moved to the Chevy section.
As Dan indicated, one side is misfiring. You only have one coil, so maybe not coil related. I imagine you have the 4.3l. Lets look outside the box. The 4.3L all have their LIM gaskets fail. Are you experiencing a coolant loss? Also, the easy stuff, again, as Dan mentioned (plugs, wires rotor, distributor cap). I had two cylinder misfire from oil dripping on plug wires that, over time caused a misfire as the oil got into the plug wires at the point of connectivity to the plug.
Lets us know more of what you have done so we can better isolate the issue.
2001 Bonneville SSEi, 219,000 miles.
Whats in my driveway:
1998 Chevy S10 4x4 ZR2, 1999 Olds Bravada, 2001 Pontiac Bonneville SSEi, 2007 Ford F-350 regular cab 2WD, 2008 BMW 335i coupe (Wife'* replacement for her Bonneville). The Bonneville is my favorite of all the cars in the driveway.