Hi John,
As you'll appreciate, there are lots of things it could be but the common, easlily-repaired ones are:
1) Short circuiting HT Leads (should not be this as you have replaced).
2) Loose Spark Plugs - remove each one and thoroughly clean the spring washer and engine block where they are seated.
3) Failing Spark Plugs - replace if cracked or where the electrodes are worn or damaged
4) Mal-adjusted Spark Plugs - adjust the gap between the electrodes to the correct settings using a Feeler Gauge.
5) Open the Distributor Cap (the other end of the HT Leads) and thoroughly clean the inside, also removing any 'crust' on the electrodes inside. Clean the distributor arm metal parts.
You can't do any harm by going through the above and it might just solve the problem.
Good luck.
Barry, November 2010
