It will be one of two things.
1. your crank position sensor wil be playing up, starting to intermittantly open circuit.
This problem won't be so easy to diagnose at home.
2, the more likely scenario is that you will have a high tension lead that will be starting to track.
Remove the engine cover, and remove all of the leads, one at a time so you don't get them mixed up! Then inspect each lead carefully. If the lead is faulty you will notice a mark on the lead tip or shaft that is surrounded by a white powder type substance. This is where the spark is "leaking".
Inspect all of the leads for this type of fault.
If you cannot detect any fault using this method, then you need to follow the next procedure.
Start the engine and allow to run until it begins to misfire, now individually pull each spark plug lead off the spark plug just a bit.
You will notice that the car either runs worse, or the same. If you pull the plug lead and it runs the same, then you have a problem with that lead or spark plug, comletely inspect both. If the spark plug is at fault then you will have black burn marks running down the porcelin of the plug, if it is the lead you will have the burn marks on the plug lead which will be surrounded by the white powder.
Good luck.
