E3 plugs

If installing new plugs made any noticeable difference, you needed new plugs. You get ONE spark, and it jumps through the path of least resistance. A sharp edge (new plug) helps a spark to form. Yes, some fancy plugs have more sharp edges, and so it may seem that they would stay efficient longer. This assumes that all those sharp edges don't erode at the same rate due to combustion chamber heat. A mild steel electrode plug lasts about 10,000 miles regardless of brand.