Something like an intake leak or stuck parially open EGR valve often cause P0300. If idle vaccum is low and runs rough but runs well at 2500rpm then I'd say EGR but if it is poor all across the line than I'd look at cap and rotor or intake leak. Ensure that all plug wires are tight.

 hi oko3 How many miles on the truck? What does it have for an engine? Any other codes? Can you isolate a cylinder? Look for tsb and service info. I would not bother with the cap & rotor since its been about 15 years since we had them. Watch your fuel trims cold and look for a lean condition.

Smoke test