GMC Repair: GMC Safari engine misfire, gmc safari, car mechanic

Question
I have question about engine misfire at 99 GMC Safari,V6 2WD,4.3L. I got check engine light permanently staying on and flashing when speed over 80mph. I already visit a car mechanic and he found P0300 code. So i changed plugs, distributor cap and rotor but the car still run very rough when accelerating. I found at Internet many people with similar problems but every one write something different. And i don´t want spend thousands of dollars(or Euros because i took the car to Germany) just to change all of the parts they causes the problem. Plz Help...

Answer
Hi
The light blinks because it is Damaging the catalyst (Catalytic Converter).
If the mechanic could look at the history misfire data, and determine WHICH cylinder OR bank of cylinders is misfiring the most.
This will aid in repair(s).
All I can give is advice at this point. From most likely..
1) an injector (determined by which cylinder is misfiring most)
2)Plug wires
3) A Engine mechanical problem (burned valve, weak spring)etc..
4)a different electrical problem (wire harness damage, etc...
5) If misfiring on all cylinders on specific bank eg..#1,#3,#5 misfiring, It COULD be the FRONT oxygen sensor for that bank of cylinders.
There are a few other items, but this may help get you started.
Good Luck
Post back, if needed.
Pawl