Buy plugs that are 0.8 rather than regapping 1.1's (most 0.8's are the same plug number with a -8 at the end). Many misfire problems are caused by aged coils, spend $500 on a set of splitfires and you will be amazed at the difference.
On a side note, when I changed the plugs in my Toyota Hiace at 100,000km the guy tried to sell me Iridiums as they are the factory plug that Toyota uses due to the difficult location on the engine. I handed them back and bought a set of coppers and ran them for an entire 100,000km without any issues at all. I've now run a second set of coppers for 103,000km without any problems