Nissan Repair: Xterra Misfire, nissan xterra, volt ohm meter

Question
QUESTION: My 2001 Nissan Xterra has a P0300 code.  A friend changed the spark pluges, distributor cap after the engine light was came on and the engine was shaking.  Weeks a go it was only when the car started and now it is constant.  The dealership was telling me that it is because I didn't use Nissan parts and wanted to replace all the new parts with theirs.  Didn't agree with that so I didn't have work on my car.   Can you suggest what it might be ?
ANSWER: Sloane,

P0300 is a random misfire being detected.  The code was there before the parts were changed?  If so, I would be looking at the crank trigger and distributor.  Now, I would look at the sparkplugs and make sure they are not cracked and gapped properly.  Lastly, I would make sure the fuel is good quality.

---------- FOLLOW-UP ----------

QUESTION: Code was there before parts were changed and is still there now (spark plugs changed and gapped to specs.)  Thanks for the suggestion I will it pass it along.

Answer
Sloane,

If it was there before I would be doing a diagnostics on the coil to make sure it works properly.  You can get a Haynes Manual and it gives you the values and how to do the test.  The book is $20 bucks as well as another $20 for the volt/ohm meter.